Intravascular imaging apparatus and methods for use and manufacture

ABSTRACT

A device for ultrasonic imaging, and methods for the use an manufacture thereof, particularly of small coronary vessels. The device comprises an elongate member with a distal end that can be positioned within a small vessel of a patient&#39;s body while a proximal end is located outside the body, a transducer located at a distal end of the elongate member and operable to scan the distal coronary vessels with ultrasonic pulses, and a signal processor connected to a proximal end of the elongate member and to the transducer for generating and receiving pulses to and from the transducer. A motor may be also connected to the proximal end of the elongate member for rotating the transducer.

REFERENCE TO RELATED APPLICATIONS

The present application is a continuation in part of Ser. No. 07/668,919filed Mar. 13, 1991 now U.S. Pat. No. 5,353,798.

BACKGROUND OF THE INVENTION

This invention relates to an ultrasonic imaging device and methods foruse and manufacture thereof, and particularly to an ultrasonic imagingdevice positionable in coronary vessels to obtain images thereof.

Ultrasonic imaging of portions of a patient's body provides a usefultool in various areas of medical practice for determining the best typeand course of treatment. Imaging of the coronary vessels of a patient byultrasonic techniques could provide physicians with valuable informationabout the extent of a stenosis in the patient and help in determiningwhether procedures such as angioplasty or atherectomy are indicated orwhether more invasive procedures may be warranted. However, obtainingultrasonic images of the distal coronary vessels with sufficiently highresolution to be valuable for making medical decisions, such asdescribed above, requires overcoming several significant obstacles oneof the most significant of which relates to the size of the ultrasonicsensing device.

Obtaining ultrasonic images of high resolution of a body organ generallyrequires bringing an ultrasonic sensor (i.e. a transmitter/receiver)sufficiently proximate to the organ and scanning the organ withultrasonic pulses. Ultrasonic imaging of organs deep within the bodythat are surrounded by other, relatively dense organs and tissuesrequires connecting a sensor on a probe and positioning the sensor andthe probe near or even into the organ. The heart and the vesselsconnected to it are organs of this type. Because it is a well knowntechnique to insert catheters, guide wires and probes into the coronaryvasculature from remote sites via arteries, such as the femoral artery,and further because some of the information of interest to the physicianis the extent of stenosis on the inside walls of the coronary vessels,it would be desirable to be able to position an ultrasonic sensorconnected to a probe into the distal regions of the coronary vasculaturevia a remote arterial site, such as the femoral artery, to obtainultrasonic images of the coronary arterial walls.

The vessels in the distal regions of the vascular tract that would beuseful to image include the coronary arteries, branch vessels stemmingfrom the external carotid artery such as the occipital and the arteriesleading to the vessels of the head and brain, splenic, and the inferiormesenteric and renal arteries leading to the organs of the thorax. To bepositioned in these regions, the size of an ultrasonic sensor and probemust be relatively small not just to traverse the arterial vessel butalso to avoid occluding the vessel lumen. When a device, such as acatheter, probe, or sensor, is positioned in a blood vessel, it occupiesa volume which restricts blood flow within the vessel as well as invessels proximate thereto. When a device is positioned within anarterial vessel, the blood flow through the vessel is restricted to anannular region (i.e. the area of "ring"-shaped cross section) which iseffectively created between the outer perimeter of the device and theinner wall of the vessel. This would normally not present a problem inlarge arteries with large blood flows, such as the femoral arteries ofthe legs, or the aorta, or in very proximal coronary arteries. In theselarge arteries, any restriction caused by the device would be relativelysmall and the blood flow would be relatively large. However, in smallarteries in remote locations, such as the occipital that leads to thebrain, or the coronary arteries of sizes of 3.0 mm or less that lead tothe right and left sides of the heart, any restriction of blood flowmust be minimized. The consequences of occluding these small vessels cancause a loss of flow in the coronary arteries of the heart which mayhave several adverse effects, such as severe chest pains, orphysiological changes such as arrythmia, ischemia, and tachycardiacresponse. These effects may be threatening to the patient and further,once begun, may be difficult to stabilize.

Moreover, not only are these latter vessels very small but these vesselsare also those in which there might also be restrictive disorders, suchas atherosclerosis. Atherosclerotic disease as well as other thrombusformations which occlude blood flow occurs in these smaller arteries dueto the hemodynamics of the blood tissue interface. Reflecting this factis that presently angioplasty is primarily performed in vessels of asize range of 2.0 to 3.5 mm in diameter. Such disorders would diminishthe cross sectional area of these vessel lumens even more.

Therefore, a significant obstacle to using an intravascular probe deviceto obtain ultrasonic images of such vessels is that the probe should besufficiently small in dimension so as not only to be positioned in thesesmall, possibly partially occluded arteries, but also to be sufficientlysmall so as not to totally or almost totally occlude the lumen of thevessel into which it is positioned. Accordingly, for an ultrasonicsensor device to be used for distal coronary applications, it must besmall enough to be suitably positioned in the coronary vessels and topermit a sufficient blood flow therearound. For use in the distalcoronary vasculature, the exterior dimension for a sensor device shouldbe approximately 0.040 inches (1 mm) in diameter to provide an annularregion of flow in even the most distal vessels.

Ultrasonic imaging devices intended to be placed in the vascular systemhave been disclosed in prior patents (e.g. U.S. Pat. No. 4,794,931).However, these prior devices have had numerous drawbacks that limitedtheir utility for the most part to uses in only the peripheralvasculature and not in deep coronary arteries. Prior devices, havingdiameters ranging from 3.5 French (1.2 mm) and up, would be limited bytheir size to only very proximal coronary arteries. Prior devices,having diameters ranging from 4.5 French (1.5 mm) and up, would belimited by their size to only very proximal locations of coronaryarteries, peripheral vessels, or very proximal organ vessels.Furthermore, in addition to these limitations, prior ultrasonic probedevices have produced images lacking in sufficiently high detail andresolution to provide useful information.

There are significant obstacles to making an ultrasonic probe devicewith dimensions sufficiently small to fit into distal coronary vesselsand yet possessing the necessary mechanical and electrical propertiesrequired for high quality ultrasonic images. For example, in order toposition a probe device in a deep coronary vessel from a remotepercutaneous site such as via the femoral artery, the probe deviceshould possess a high degree of longitudinal flexibility over itslength. The vessel paths of access to such deep coronary vessels, aswell as the numerous branches which stem from these vessels, may be ofan extremely tortuous nature. In some areas within the vascular system,an ultrasonic probe device may have to transverse several curvatures ofradius of 3/16 of an inch (4.7 mm) or less. Thus, the probe deviceshould possess a high degree of flexibility longitudinally over itslength to enable it to transverse virtually any curvature of thevascular tract.

Another desired mechanical property for the probe device is stabletorsional transmittance. If the device is to include a rotatingultrasonic sensor at a distal end to make radial scans of the entirecross section of the coronary artery, it should not only be flexiblelongitudinally, but should also be stiff torsionally. Rotation of theultrasonic device should be achieved so that a drive shaft extending tothe sensor does not experience angular deflection which might causeimage distortion. Due to the continuous angular motion which dictatesthe location at which an ultrasound sensor scans, if angular deflectionoccurs at the distal end of sensor drive shaft, it can result in anartifact of angular distortion that becomes apparent on the ultrasounddisplayed image. This artifact can occur if the rotating sensor driveshaft experiences "whip". "Whip" may be defined as the angulardeceleration and acceleration of the sensor drive shaft as a result inshaft angular deflections during rotation. As the transducer drive shaftis rotating it may undergo angular deflection if the drive shaft'storsional stiffness is low enough to make the drive shaft susceptible todynamic changes in torsional load. It may also undergo angulardeflection if the dynamic torsional loads are high and varying.

During operation, relative changes in torsional load should be minimal,therefore any induced `whip` could be attributed to a shaft with a lowtorsional stiffness. The acceleration and decelerations associated withshaft whip can be described by the energy change from kinetic topotential under varying torsional load conditions. For example, as asensor drive shaft encounters additional torsional resistance itsangular velocity drops causing a deceleration and shaft angulardeflection. When the shaft is free of the added resistance, the energystored in the shaft, in the form of potential energy from the angulardeflection and shaft stiffness, is released causing an angularacceleration and increase in the shaft's angular velocity.

Image quality and accuracy is dependent on constant sensor angularvelocity. Image construction assumes a constant sensor velocity,therefore relative acceleration or deceleration between the expected andactual sensor angular velocity will cause image distortion.

Even if a sensor possesses the aforementioned minimal size andmechanical properties, the value of the device depends upon the qualityof the ultrasonic image which in turn is a direct function of both theacoustic pulse signal and the electrical signal transmission. Therefore,in addition to the mechanical properties necessary for locating androtating a sensor, the device should also provide a high qualityelectrical and acoustic signal. This may include several specificproperties, such as a high signal to noise ratio of the electronicsignal, impedance matching of the overall system without the need forinternal electronic matching components, and minimization of voltagerequirements to attain a signal of sufficient quality to provide animage.

Accordingly, it is an object of the present invention to provide adevice that overcomes the limitations of the prior art and which enablesthe ultrasonic scanning of small diameter body vessels with a transducerprobe that can be positioned therein.

It is a further object of the invention to provide an apparatus, andmethods for use and manufacture, that enables obtaining ultrasonic imageinformation of high resolution or quality.

SUMMARY OF THE INVENTION

The present invention provides a device for intravascular ultrasonicimaging, and methods for the use an manufacture thereof, comprising anelongate member with a distal end that can be positioned within a vesselof a patient's body while a proximal end is positionable outside thebody. The device also includes a transducer located at a distal end ofthe elongate member and a signal processor connected to a proximal endof the elongate member for generating pulses to and receiving from saidtransducer. The device preferably includes a motor for rotating thetransducer and a drive cable for connecting the transducer to the motorand the signal processor. The drive cable is operable to transmitelectrical signals to and from the transducer.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of a presently preferred embodimentof the ultrasonic imaging apparatus.

FIG. 2 is a longitudinal vertical sectional view of a distal portion ofthe ultrasonic imaging apparatus depicted in FIG. 1.

FIG. 3 is a sectional view of the distal portion of the ultrasonicimaging apparatus along lines 3--3' in FIG. 2.

FIG. 4 is a sectional view of the distal portion of the ultrasonicimaging apparatus along lines 4--4' in FIG. 2.

FIG. 5 is a plan view of a portion, partially disassembled, of the drivecable.

FIG. 6 is a sectional view of an embodiment of the elongate member ofthe system depicted in FIG. 1.

FIG. 7a a sectional view alone lines 7--7' of the embodiment of theelongate member depicted in FIG. 6 illustrating a first alternativeindexing function construction.

FIG. 7b a sectional view alone lines 7--7' of the embodiment of theelongate member depicted in FIG. 6 illustrating a second alternativeindexing function construction.

FIGS. 8a and 8b are block diagrams of processing steps related toacoustical indexing.

FIG. 9 is a sectional view of the alternative embodiment of the elongatemember shown in FIG. 6 illustrating a first flushing method.

FIG. 10 is a sectional view of a second alternative embodiment of theelongate member illustrating a second flushing method.

FIG. 11 is a sectional view along lines 10--10' of the embodiment inFIG. 9.

FIG. 12 is a sectional view of a portion of a third alternativeembodiment of the elongate member illustrating a third flushing method.

FIG. 13 is a plan view of the uncoupling member shown in FIG. 1.

FIG. 14 is a longitudinal vertical sectional view of the transducer pinassembly shown in FIG. 13.

FIG. 15 is a longitudinal vertical sectional view of the slip ringassembly shown in FIG. 13.

FIG. 16 is a plan view with a partial sectional view of the proximaldrive cable shown in FIG. 1.

FIG. 17 is a diagram of signal amplitude versus time for the pulser in afirst embodiment of the present invention.

FIG. 18 is a diagram of signal intensity versus radial distance from thesensor perpendicular to drive direction.

FIG. 19 is a diagram of signal intensity versus radial distance from thesensor perpendicular to drive direction.

FIG. 20 is a diagram of signal intensity versus the position along thecross section A--A' of FIG. 18.

FIG. 21 is a diagram of signal intensity versus the position along thecross section B--B' of FIG. 18.

FIG. 22 is a block diagram of the calibrated waveform of the pulser.

FIG. 23 is a perspective view of another embodiment of the transducersensor.

FIG. 24 is a plan view of the distal end of an imaging guide wire whichis another embodiment of the present invention.

FIG. 25 is plan view of yet another embodiment of the sensor housing ofthe present invention.

FIG. 26 is plan view of still another embodiment of the sensor housingof the present invention.

FIG. 27 is plan view of another embodiment of the present invention for3-D imaging.

FIG. 28 is a view of a distal section of an alternative embodiment ofthe elongate member with variations represented for 3-D indexing.

FIG. 29 is a cross sectional view of the embodiment shown in FIG. 28along lines A--A'.

FIG. 30 is a top view of a the distal end of yet another embodiment ofthe present invention that utilizes an alternative drive mechanism.

FIG. 31 is an alternative embodiment of that shown in FIG. 30.

FIG. 32 is a block diagram of the data and graphics pipeline of analternative embodiment of the present invention.

FIG. 33 is a diagram illustrating utilization of a neural networkarchitecture in an alternative embodiment of the present invention.

FIG. 35 is a side elevational view of a first preferred embodiment of animaging guide wire.

FIG. 36 is a side elevational view of a preferred embodiment of a slicedtransducer sensor for use in the imaging guide wire of FIG. 35.

FIG. 36a is a cross sectional view of the sliced transducer sensor ofFIG. 36.

FIG. 37 is a top view of the sliced transducer sensor of FIGS. 36 and36a.

FIGS. 36, 38, and 39 each show a top view of alternative constructionsof the sliced transducer sensor of FIGS. 36 and 36a.

FIG. 40 is a side elevational view of the preferred embodiment of thetransducer sensor for use in the imaging guide wire of FIG. 35incorporating a sheath over the transducer sensor.

FIG. 40a is a cross sectional view along line A--A' of the transducersensor of FIG. 40.

FIG. 41 is a side elevational view of an alternative embodiment of thetransducer sensor for use in the imaging guide wire of FIG. 35incorporating an exponential matching layer.

FIG. 41a is a cross sectional view along line A--A' of the transducersensor of FIG. 41.

FIG. 42 is a side elevational view of a preferred embodiment of thetransducer sensor for use in the imaging guide wire of FIG. 35incorporating a formed sheath matching layer.

FIG. 42a is a cross sectional view along line A--A' of the transducersensor of FIG. 42.

FIG. 43 is a side elevational view of an embodiment of the transducersensor for use in the imaging guide wire of FIG. 35 incorporating asplined attenuation backing support.

FIG. 43a is a cross sectional view along line A--A' of the transducersensor of FIG. 43.

FIG. 44 is a side elevational view of an embodiment of a wedgetransducer sensor for use in the imaging guide wire of FIG. 35.

FIG. 44a is a cross sectional view along line A--A' of the transducersensor of FIG. 44.

FIG. 45 is a side elevational view of an embodiment of a multipletransducer sensor for use in the imaging guide wire of FIG. 35.

FIG. 45a is a cross sectional view along line A--A' of the transducersensor of FIG. 45.

FIG. 46 is a side elevational view of an embodiment of the distal tipconstruction of the imaging guide wire of FIG. 35.

FIG. 47 is a side elevational view of an alternative embodiment of thedistal tip construction of the imaging guide wire of FIG. 35incorporating a locking tip feature.

FIG. 48 is a perspective view, partially disassembled, of an embodimentof the drive cable construction of the imaging guide wire of FIG. 35.

FIGS. 49, 50, and 51 each show a perspective view of alternativeembodiments of the proximal end section of the imaging guide wire ofFIG. 35.

FIG. 52 is a side elevational view of an extension wire for use with theimaging guide wire of FIG. 35.

FIG. 53 is a side sectional view of a drive interface for making theelectrical an mechanical connections for driving the imaging guide wireof FIG. 35.

FIGS. 54a and 54b each show alternative embodiments of supporting meansfor the proximal end section of the imaging guide wire of FIG. 35.

FIG. 55 is a side sectional view of a holder apparatus for the imagingguide wire of FIG. 35.

FIG. 56 is a flow chart representing an embodiment of the pipelinearchitecture for the imager of FIGS. 1 or 35.

FIG. 57 is a side sectional view of an alternative embodiment of theslip ring assembly of FIG. 15 incorporating a capacitive non-contactingslip ring assembly.

FIG. 58 is a side sectional view of an alternative embodiment of theslip ring assembly of FIG. 15 incorporating a magnetic non-contactingslip ring assembly.

FIG. 59 is a side sectional view of an alternative embodiment of theimager of FIGS. 1 or 35 incorporating an EEPROM into the imager to storeessential product information.

FIG. 60 is a perspective view of an embodiment of a cath lab patienttable and accessories for use with the imager of FIGS. 1 or 35.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS I. THESYSTEM

Referring to FIG. 1, there is depicted a schematic representation anultrasonic imaging system 20. The system comprises a sensor assembly 24located at a distal end of the system 20 at a distal end of an elongatemember 26. The elongate member 26 can be percutaneously positioned inthe cardiovascular system of a patient via a remote site such as thefemoral artery so the distal end of the elongate member is located in orclose to the remote site while a proximal end extends out the body ofthe patient. The elongate member 26 includes at a distal end thereof thesensor assembly 24. The elongate member 26 further includes means fortransmitting an electrical signal between the sensor assembly 24 locatedat the distal end thereof and the proximal end extending out of the bodyof the patient. The elongate member 26 further includes means foroperating the sensor assembly to make scans of the remote vessel site.In a preferred embodiment, the means for operating the sensor assembly24 and the means for transmitting a electrical signal to and from it areprovided by a distal drive cable 28 located inside the elongate member26. The sensor assembly 24 is connected to a distal end of the distaldrive cable 28. The distal drive cable 28 is connected at its proximalend to a coupling member 30 which connects to components located at aproximal end of the system 20. Specifically, the coupling member 30serves to releasably couple the distal drive cable 28 to, and uncouplethe distal drive cable 28 from, a proximal drive cable 32. The proximaldrive cable 32 includes a first leg 33 that connects to a signalprocessing unit 34 and a second leg 35 that connects to a motor 36.Connected to both the signal processing unit 34 and the motor 36 is acontrol unit 38 that serves to operate the motor 36 and the signalprocessing unit 34. These components are described in further detailbelow.

This embodiment of the present invention is particularly adapted forultrasonic diagnostic imaging in the small, distal vessels of a humanpatient. These vessels typically have diameters of only up to 4.5 mmdiameter. In particular, the present embodiment is adapted for use indeep organ vessels where the residual diameter of the vessel may be 1.5mm or less. However, it should be understood that embodiments of presentinvention may be readily adapted for use in vessels having otherdimensions with corresponding advantages in these other size vessels aswell. In the preferred embodiment for use in vessels having a diameterof approximately 3.5 mm with potential stenosis resulting in diametersof down to 1.2 mm, the overall maximum diameter of the distal portion ofthe ultrasound imaging system is preferably not more than approximately3.2 French (1.07 mm or 0.42 inch) and preferably the distal portion ofthe system has an overall diameter of less than 3.0 French (1.0 mm).

In operation, the signal processing unit 34 generates electrical pulsesthat are transmitted to the sensor assembly 24 via a proximal electricaltransmission cable inside the proximal drive cable 32 (as furtherdescribed below) and the distal drive cable 28. The signal processingunit 34 also receives electrical pulses back from the sensor assembly 24via these cables. At the same time, the motor 36 operates to rotate aproximal drive shaft located inside the proximal drive cable 32 (asdescribed below) which in turn rotates the sensor assembly 24 to whichit is coupled via the distal drive cable 28. Rotation of the sensorassembly 24 while pulsing and receiving the reflections effects anradial ultrasonic scan of the area proximate to the sensor assembly 24.In this embodiment, the motor 36 operates to rotate the transducerassembly 24 at speeds ranging from 500 to 1800 RPM, with a preferredrotational speed of approximately 1000 RPM.

The design and construction of the various components of the system arepreferably computer modeled and iterated to provide optimum overallsystem performance. For example, for optimum performance, impedancethroughout the overall system from the signal processing unit 34 to thesensor assembly 24 is carefully matched to eliminate reflections at allinterfaces caused by impedance mismatch. By eliminating reflections inthe system, there is a faster settling of the pulses since reflectionscan cause ringing of the pulse thus reducing the radial resolution.Because there is limited potential for adjustment of the impedance atthe sensor assembly 24 end of the system, consistent with otherrequirements, the rest of the system components proximal from the sensorassembly 24 are matched to it. In this embodiment, a system impedance of50 ohms is selected. With a system impedance of 50 ohms, readilyavailable industry standard components, such as coaxial cables may beused for proximal equipment. A suitable sensor can be constructed andused that is matched to this impedance and that has an active surfacearea of 0.50 mm². Similarly, the distal drive cable 28 and the proximaldrive cable 32 are constructed with an impedance of 50 ohms. Theimpedance of the coupling member 30 is not specifically matched to thatof the rest of the system. The coupling member has a low resistance,e.g. less than 0.5 ohm. However, the length of the unmatched impedanceportion of the coupling member is made to be only approximately 0.75inch. At the preferred operating frequency of 30 Mhz, a segment of thislength with an unmatched impedance can be present in the electricaltransmission conductor of the system without causing a significantreflection. The signal processing unit 34 (including the pulser), atsignal voltage levels, is also selected with impedance matched to thesystem impedance, i.e. 50 ohms, to eliminate reflections. With a matchedtermination at the signal processing end of the system, the signal isinsensitive to the length of the cable members. This provides theadvantage that the motor 36 and signal processing unit can be positionedout of the way of the physician, e.g. under a table or other convenientplace.

II. THE SENSOR ASSEMBLY

Referring to FIG. 2, there is depicted a vertical longitudinal sectionalview of a distal portion of the imaging system 20 including the sensorassembly 24 of a first presently preferred embodiment. The sensorassembly 24 is located inside the elongate member 26. The sensorassembly 24 is connected at a proximal end thereof to the drive cable28.

The sensor assembly 24 includes a sensor housing 40 in which is mounteda transducer sensor 42. The transducer housing 40 is a hollow, generallytubular member having a cylindrical wall and open ends. The housing 40has dimensions that provide for positioning and rotating inside of alumen 43 of the elongate tubular member 26. In a preferred embodiment,the housing 40 has an outside diameter of 0.030 inches. This may beequal to the diameter of the drive cable 28. In a preferred embodiment,the housing 40 is a metallic tube of 304 stainless steel.

The transducer sensor operates in alternating pulsing and sensing modes.In the pulsing mode, when excited electrically, the transducer sensor 42creates a pressure wave pulse which travels through the elongate memberinto the arterial environment. In the sensing mode, the transducersensor 42 produces an electrical signal as a result of receivingpressure waves reflected back to the transducer. These reflections aregenerated by the pressure waves traveling through changes in density inthe arterial environment being imaged. The electrical signals producedby the transducer sensor 42 are transmitted back to the signalprocessing unit 34 for generation of images of the arterial environmentby methods known in the art and as further described below.

Referring to FIGS. 2 and 3, the transducer sensor 42 is constructed fromseveral distinct layers including a transducer core material 44 having afirst and a second metallized electrically conductive surface layers,45a and 45b, bonded thereto, a matching layer 46, a backing layer 47bonded to the metallized surfaces, and one or more adhesive layers. Thisconstruction provides a transducer sensor with an active area ofapproximately 1.0×0.5 mm. The impedance of the transducer is a linearfunction of the active area so for a device having an active surfacearea of about 1.0×0.5 mm, the impedance is approximately 50 ohm.

Transducer Sensor Core Material

In a preferred embodiment, the transducer core 44 of the transducersensor 42 is a flat rectangular piece of PZT (Lead Zirconate Titarate)type ceramic material. Such PZT material has an acoustic impedance ofmid 20's and a speed of sound of about 5000 m/s. At this speed, thethickness for a 30 MHz sensor is about 0.003 inch. At this thickness,PZT materials should be selected with small grain sizes so that shortsare not generated during processing. The PZT material is cut to arectangular shape of 0.5×1.25 mm. The active area after wires and amatching layer are attached is approximately 0.5×1.0 mm.

Transducer Sensor Conductive Layers

The first and second conductive layers, 45a and 45b, are positionedrespectively on each face of the transducer core 44. The conductivelayers 45a and 45b may be composed of a number of electricallyconductive materials, such as gold, silver, copper, or nickel. However,a number of other materials, elements or alloys are suitable. Additionallayers may be needed under each conductive layer to provide for adhesionto the core material, e.g., using chromium under gold. For goodperformance, the resistance of the conductive layers should be less than1 ohm from one end thereof to the other.

Transducer Sensor Matching Layer

The matching layer 46 provides an impedance transformation between thetransducer sensor 42 and the fluid therearound to allow a bettercoupling of energy into the fluid. This transformation is frequencydependent. A matching layer may be used where a difference existsbetween the transducer and the medium adjacent thereto. Use of amatching layer provides for a stronger and sharper pulse and thus abetter image. The optimized value range for the matching layer is from3.8 to 4.2 (×10⁶ kg/m² sec.). The material that is used for the matchinglayer may be PVDF (Kynar) at a thickness of 0.95× (quarter wavelengththickness). The matching layer 46 is bonded to the first conductivelayer 45a by means of a thin glue layer. The matching layer 46 conformsapproximately in surface dimension to the active size of the transducer,i.e. 0.5×1.0 mm.

Transducer Sensor Backing Layer

Bonded to the conductive layer 45b on the opposite surface of the core44 from the matching layer 46 is the backing layer 47. The backing layerserves to absorb acoustical energy generated off the non-imaging side ofthe transducer and also helps minimize energy reflections coming back tothe transducer. The amount of energy traveling from the transducer coreto the backing is a function of the acoustic impedance of the core andthe backing material. The energy that is generated and enters thebacking material should be attenuated sufficiently before it isreflected back into the core where it can distort the signal. Thebacking layer 47 impedance is selected to provide optimum damping sothat the transducer sensor 42 vibrates for only a short duration afterelectrical excitation is stopped and prevents energy from beingreflected to or from the artery wall to the back side of the transducer.This enables the transducer sensor 42 to be ready to receive pressurewaves reflected from the arterial environment with no or minimalinterference from ringing from the pulse. The impedance of the backinglayer may be determined by computer modeling and in this embodiment isselected in the range from 5 to 7 (×10⁶ kg/m² sec.). The compositionused for the backing is preferably a tungsten and silicon rubbermixture. The acoustical impedance of the mixture can be varied by mixingvarious sized tungsten powder particles into the silicon rubber. Thismixture is very good for backing since it has very high attenuation. Thebacking layer 47 may be bonded to the conductive surface 45b by means ofa thin glue layer applied on backing type material.

The backing layer 47 conforms in surface dimension to the size of theactive area, i.e. approximately 0.5×1.0 mm. In order to allow sufficientringdown after pulsing, the backing layer 47 is preferably provided witha maximum thickness, or depth dimension, consistent with the dimensionsof the sensor housing 40, drive cable, elongate member, etc. As shown inFIG. 3, in the present embodiment, the backing layer 47 may be made to adimension equal to the cross-section of the drive cable 28 and/orhousing 40. This allows for a backing layer of a maximum size to providefor sensor ringdown time and yet is small enough to fit deep into thecoronary arterial environment. The backing layer may be approximately0.012 inches in thickness.

The transducer sensor 42 is connected by the sides 48 and 49 thereof tothe interior of wall 50 of the housing 40. The transducer sensor 42 ismounted so that the central axis of the sensor assembly 24 passesthrough or is close to the plane defined by the flat surface of thetransducer sensor 42. Thus, the flat surface of the transducer facesperpendicular to its axis of rotation. This permits maximizing thedimensions of the matching layer and backing layer. This constructionalso allows for secure mounting of the sensor assembly 24 to the drivecable 28 by inserting and connecting the housing 40 to the distal end ofthe drive cable 28.

The housing 40 has a first acoustic window (or aperture) 52 and a secondwindow 53 oppositely located from each other in the cylindrical wall ofthe housing 40. These windows are preferably approximately rectangularin shape having parallel sides in the longitudinal direction of thehousing 40 and rounded sides in the chordal direction. These windows maybe formed by removing portions of the material of the cylindrical wallof the housing but leaving narrow bands 54 and 55 of the wall 50 of thehousing 40 onto which the transducer sides 48 and 49 may be bonded. In apreferred embodiment, both windows 52 and 53 are approximately 0.6×2.0mm. In the sensor assembly 24, the transducer sensor 42 is mounted andlocated in the housing 40 directly facing the first window 52 so thatthe ultrasonic signal is emitted from the transducer sensor 42 throughthe first window 52.

The size and geometry of the windows are related to the pulse generatingcharacteristics and the advantages of the disclosed window geometry aredescribed below in conjunction with the description of the operation ofthe pulser.

These windows 52 and 53 may also be useful during the construction andtesting of the sensor assembly 24. The sensor assembly 24 can beconstructed and tested before mounting to the drive cable 28 byconnecting the wires between the tested sensor and the tested cableinside the window. This ability to screen sensor assemblies prior toattachment to the drive cable increases transducer drive shaft assemblyyield dramatically. Also, the housing design also allows alignment ofthe transducer in the elongate member during rotation by the smoothrounded end and fit between elongate member 26 and the housing 40.

Referring to FIG. 4, the sensor assembly 24 is connected at its proximalend to the distal end of the drive cable 28. Specifically, the firstconductive layer 45a of the transducer sensor 42 is connected to thedistal end of an internal conductor 58 of the drive cable core wire 60.A distal end of an external layered coil portion 62 of the drive cable28 is connected to the housing 40. These connections may be made bymeans of an epoxy adhesive. An external conductor 63 (also referred toas the reference plane conductor) of the core wire 60 is sealed by meansof an epoxy. The reference plane conductor 63 of the core wire 60 isconnected electrically to the housing 40 via the external layered coilportion 62 of the drive cable 28.

In a preferred embodiment, a single transducer is mounted in a singletransducer housing which is connected at the distal end of a drivecable. However, in other embodiments, as described below, more than onetransducer with one or more housings, may be connected serially at theend of a drive cable in order to make scans of a length of a vessel. Insuch multi-transducer embodiments, an appropriate switching device maybe utilized in conjunction with the signal processing unit and thetransducers to coordinate pulsing and receiving data.

III. DRIVE CABLE

Referring to FIG. 5, there is depicted a portion of the drive cable 28,partially disassembled. In the assembled imaging system 20, the drivecable 28 is positioned inside the elongate member 26 and is connected tothe sensor assembly 24, as described above. The drive cable 28 serves asboth the mechanical and electrical link to the sensor assembly 24.

The drive cable 28 conducts the electrical signal from the proximallylocated signal processing unit 34 (via the proximal drive cable 32) tothe sensor assembly 24 and conveys the sensed signal from the sensorassembly 24 back to the signal processing unit 34. In order to provide adrive cable of a suitably minimal dimension for coronary applicationswhile providing both the necessary mechanical and electrical properties,the electrical components of the drive cable provide for mechanicalmotion transmittance as well. Thus, the drive cable 28 connects thesensor assembly 24 to the proximally located motor 36, via a drive shaftlocated in the proximal drive cable 32, in order to rotate the sensorassembly 24 to scan the coronary vasculature with an ultrasonic signal.

In order to provide high quality electrical signal transmission, thedrive cable 28 possesses a controlled matched impedance, a low signalloss, and high shielding and conductivity at high frequencies. Asmentioned above, the need for a matched impedance in the drive cable 28follows from the requirement for matching impedances at interfaces ofthe overall imaging system from the signal processing unit 34 to thesensor assembly 24 in order to eliminate reflections. Because of therelative difficulty in adjusting the impedance at the sensor assembly 24end of the system, the rest of the system components, including thedrive cable 28, are matched to that of the impedance of the transducersensor 42. Accordingly, the impedance of the drive cable 28 is matchedto that of the sensor assembly 24 and in this embodiment is establishedto be 50 ohms.

Mechanically, the drive cable 28 possesses high torsional stiffness(i.e. minimal angular deflection under operating torsional load) yetpossess longitudinal (axial) flexibility to allow percutaneouspositioning in the coronary vessels. In addition, as mentioned above,the drive cable 28 also possesses dimensional properties suitable forpositioning in a patient's coronary vasculature, specifically the drivecable 28 has a low profile diameter to navigate torturous coronaryarteries. A present embodiment provides these features in part by acoaxial multi-layer drive cable construction. The drive cable 28includes a core wire 60 located inside of an outer layered coil assembly62, as explained below.

The core wire 60 is located at the center of the drive cable 28. Thecore wire 60 includes an insulated internal conductor 58. The core wire60 has a diameter of 0.014 inch and its internal conductor 58 is 38 AWG(7 strands of 46 AWG) copper wire. The internal conductor 58 issurrounded by a teflon coating that forms an insulator layer 66. Teflonis used as an insulator for the internal conductor 58 of the core wire60 because of the relatively low dielectric constant which allows for asmaller cable, less loss, and higher speed of electrical transmissionfor a given impedance.

Around the insulated internal conductor 58 is an external conductor 63in the form of a braided shield which forms the exterior electricalshield of the core wire 60. The braided shield is preferably composed ofeight silver-plated, rectangular copper strands 70, four in eachdirection of rotation. Specifically, each strand is 0.001×0.007 inchoxygen free highly conductive (OFHC) copper with 50 micro-inches ofsilver plating.

Use of flat wire of these dimensions for the construction of the braidedshield allows excellent coverage of the core wire 60 while maintaining alow braid profile. This flat wire braid contributes only about 0.004inch to the overall cross-section of the drive cable 28. Furthermore,the 7 mill cross-sectional area of each strand provides enough strengthto form the braid with standard braiding equipment. The use of flat wirefor the braided shield of the external conductive wire 63 also providesadvantages for electrical transmission through the drive cable 28. Aflat wire braided shield with its inherently large surface area producesa conductor of low resistance (i.e. low cable loss) when compared todimensionally equivalent round wire braided shields. Because electricalcurrent travels through a braided shield following a path of leastresistance, the use of a rectangular braid for the shield provides alarge surface area at overlapping wires allowing lower resistancecontacts thereat.

Use of silver plating on the external conductive wire 63 providesseveral further advantages. First of all, the silver plating provides ahigh quality environmental seal from corrosion. In addition, the silverplating on the flat copper wires of the braided shield of the externalconductive wire 63 also advantageously reduces the shield's electricalresistance at the high electrical frequencies due to "skin effect".Electrical transmission through a conductor wire at high frequenciesexhibits a "skin effect" which is a phenomena wherein the electricalcurrent tends to increasingly travel in the outer periphery of aconductor as the signal frequency is increased. At the frequencies ofoperation of the imaging system, most of the current would be carried inthe conductor within less than 0.0005 inch of the surface of theconductor. This is one of the reasons that the external conductor wire63 is made with silver plating because silver has a lower resistivitythan copper. For a given thickness more current will be carried in asilver layer than in the copper base.

A further reason for using silver plating is its property ofnon-corrosiveness which helps maintain low electrical resistance at theoverlapping joints of the braided shield of the external conductor 63.The application of the silver plated, braided shield to the insulatedinternal wire thus forms a high quality miniature 50 ohm coaxial cablewith a total diameter less than 0.030 inch (0.75 mm).

In the drive cable 28, around the core wire 60 is located the layeredcoil assembly 62. In a preferred embodiment, the layered coil assembly62 comprises a multi-layer, multi-strand coil for optimum torquetransmittance. The layered coil assembly 62 of the present embodiment iscomprised of three layers 74, 76, and 78. Each coil layer is composed ofthree separate wires strands, e.g. coil layer 78 is comprised of strands80, 82, and 84. Each strand may be comprised of a 50 micro-inch silverplated, oxygen free highly conductive (OFHC) copper ribbon wire havingdimensions of 0.001×0.007 inch. This construction of the layered coilassembly provides for suitable torque transmission (or stiffness) byreducing the torsional load per strand.

These three layers 74, 76, and 78 are applied in opposing windingdirections to the layer immediately adjacent thereto. For example, coillayer 74 is wound in an opposite helical direction from that of coillayer 76, and coil layer 76 is wound in an opposite helical directionfrom that of coil layer 78 (but coil layer 78 would be wound in the samehelical direction as coil layer 74). The coil winding direction isdetermined so as to be consistent with the direction of drive cablerotation so that during operation of the system, the layered coilassembly will tend to tighten upon itself thereby providing additionaltorsional stiffening effects to the drive cable during operation withoutdecreasing the cable's longitudinal flexibility during positioning.Increasing the torque stiffness reduces the angular deflection per coillayer.

Again, the use of flat wire for the layered coil assembly has severaladvantages. Using flat wire helps in maintaining the low profile of thedrive cable, e.g. only approximately 0.028 inch. This is significantlysmaller than would be possible if a round wire of equivalent inertialmoment were used. In addition, the use of multiple flat wire coilsprovides a significant amount of shaft flexibility due to the inherentslip planes between coils and strands which facilitates placement of thedrive cable in the coronary arteries.

The utilization of silver plated OFHC copper for the layered coilassembly 62 advantageously benefits the drive cable's electricalproperties as well. The use of the silver plated OFHC copper providesshielding effectiveness and lower resistance than other conductors (bothDC resistance and high frequency resistance due to "skin effect" inconductors). These properties reduce the electrical signal attenuationthrough the drive cable 28 and aid in producing the cable's matchedimpedance. These electrical characteristics improve the overall systemperformance by improving the signal to noise ratio and eliminating theneed for impedance matching components.

Manufacturing Process for the Drive Cable

The drive cable 28 may be constructed according to the followingprocedure.

First, the core wire 60 is constructed. The braided shield for thereference plane conductor is constructed over a 0.014 inch diameterteflon insulated core wire using a Kokubun braiding machine. The Kokubunbraider utilizes 16 bobbins containing braid wire moving in ainter-twining planetary action to create an interlaced braid. Bobbinmovement, in terms of orbiting speed, and feed rate of the central corewire through the braiding area are controlled by two speed regulatedmotors, such as 21/4 H. P. Emerson Motors, P/N 3120-406. Motor speed ofthe core wire take up pulley and the bobbin rotation are closelyregulated to predetermined values to ensure finished shaft's mechanicaland electrical properties. This may be done with a Focus 1 SpeedController.

The following process is followed to set up the Kokubun braidingmachine. The teflon insulated internal wire 58 is routed through thecenter guide of the braider's bobbin carriage. Due to the fragility ofthe internal wire and the ribbon wire to be braided over it, anadditional core wire guiding apparatus providing wire support, backtension, and braid wire entrance angle guiding is added to the Kokubunbraiding machine. Also, the back tension provided at the bobbins for thebraid flat wire has been reduced to approximately 35% of its originalvalue. A modified upper guide has been added to control the smalldiameter braided wire's movement during the braiding process.

The Kokubun braider provides positions for 16 bobbins from which tocreate a 16 strand braid. Eight of these bobbins are removed to generatea coarser braid. The eight bobbins removed consist of 4 in eachdirection in an alternating fashion such that the remaining interleavedbraid consists of four strands in each direction.

The braiding machine is started and bobbin carriage and braid take upwheel motor speed are adjusted. The bobbin carriage speed is set to 395+/-5 RPM. The braid take up wheel speed is set to 530 +/-5 RPM. Thebraider configuration is modified such that the bobbin carriage motorhas been fitted with a 5:1 gear reducer and similarly, the braid take upwheel motor utilizes a 30:1 gear reducer to provide the appropriatecarriage and take up speeds.

The internal wire is routed through the braider's main guide and theupper broad guide and attached securely to the take up wheel.

Bobbins containing the 0.001 inch×0.007 inch silver plated OFHC copperribbon wire are threaded through the upper guide and attached to thebraider's take up wheel, one strand at a time. Using the manual carriagecrank, the bobbin carriage is rotated through 5 full rotations in orderto initiate the braid on the internal wire.

After initiation of the braid onto the internal wire, the braid isbonded to the core wire using a cyanoacrylate adhesive over the entireexisting braid length.

The braiding machine is started by simultaneously switching on both thecarriage motor and the take up wheel motor. The motor speeds areverified with respect to their preset values. The braider is thenallowed to operate for sufficient time to produce the required length ofbraided core cable based on the braider's approximate production of 0.33feet/min.

Upon completion of the braided length, the braid is bonded using acyanoacrylate adhesive over 0.5 inch bond length. The braid is cut atthe bond area and removed from the braiding machine. The core wireportion is completed.

Next, the layered coil portion 63 is added to the core wire 60. A lengthof core wire of 66 inches is provided. The core wire 60 is prepared forthe addition of the layered coil portion 62 by bonding the braided wireends of the external conductor 63 using cyanoacrylate adhesive over a0.5 inch length to prevent unravel of the braid.

The application of layered coil portion 62 to the core wire 60 isperformed using an Accuwinder Model CW-16A. The core wire is loaded into the coil winder head and tail stock chucks. Three spools of 0.001inch×0.007 inch silver plated, OFHC copper ribbon wire are loaded on thecoil winder's spool carriage. The wires are individually threadedthrough the coil winder's two guides and two tensioning clamps andfinally through the three wire, lead angle guide. Wires must be routedunder the three wire guide wheel and over the lead angle guide. Thetensioning clamps are set to light tension. The spool carriage is movedinto its initial coiling position; it is located such that the leadangle wire guide is approximately 0.25 inches (axially) from and headstock, and approximately 0.005 inches (radially) from the core wire.Guide adjustments are made by loosening their retaining screws.

The first multistrand coil 74 is wound with the coil winder's rotationdirection switch in the clockwise (CW) position. This coil windingrotation direction requires the three coil strands to be routed beneaththe core wire and secured to the head stock spindle.

The coil winding computer controller is powered on in conjunction withthe coil winder itself. Control by the computer over the coil winder isobtained by initiating the following winding parameters via the windingprogram ULTRA₋₋ SD: coil pitch=0.0232 inches, maximum winding speed=1780RPM. The lead angle at which the wire approaches the core wire iscontrolled by way of the lead angle guide and the coil pitch. Thewinding control program is down loaded to the coil winder.

Axial tension is slowly added to the core wire until a value of 3 to 5pounds-force is reached.

The operating lever is lowered. Using the speed control knob, the coilwinding speed is slowly increased to a maximum value of 60%. Core wiretension is continuously monitored during the coil winding process tomaintain a wire tension of 3-5 pounds-force.

Coil winding is continued until the lead angle guide is within 1 inch,axially, of the tail stock chuck. The coiling process is halted byraising the operating lever and reducing the speed control to 0%. Thecoils are bonded to the core wire at the head and tail stock locationover a 0.5 inch bond length. The three strands used to form the coil 74are cut at the core wire 60 and care is taken to prevent damage to thecore wire 60. The spool carriage is returned to the head stock locationin preparation for applying the second, opposing, coil 76.

The tail stock pulley is loosened such that it can move independently ofthe coil winder drive shaft. The tail stock spindle is rotated 5 fullrevolutions in the CCW direction (when viewing the front of the tailstock chuck) in order to preload the first coil. The tail stock pulleyis tightened.

The three ribbon wires to be coiled are routed under the three wireguide, over the lead angle guide, and placed over the core wire; thewires are temporarily secured to the head stock spindle. The coilwinder's rotation direction switch is moved to the Counter Clock Wise(CCW) position. The operating lever is lowered and the speed control isincreased gradually to 60%. The core wire tension is maintained at 3-5pounds-force.

Coiling is continued until the lead angle guide is within 1 foot,axially, of the tail stock chuck. The coiling process is halted byraising the operating lever and reducing the speed control to 0%. Thecoils in this layer 76 are bonded to the core wire 60 at the head andtail stock locations over a 0.5 inch bond length. The three strands usedto form the coil 76 are cut at the core wire 60 and care is taken toprevent damage to the core wire 60. The spool carriage is returned tothe head stock location in preparation for applying the third coil 78.

The tail stock pulley is loosened such that it can move independently ofthe coil winder drive shaft. The tail stock spindle is rotated 5 fullrevolutions in the CW direction (when viewing the front of the tailstock chuck) in order to preload the second coil. The tail stock pulleyis tightened.

Ribbon wires to be coiled are routed under the three wire guide, overthe lead guide, and placed under the core wire; the wires aretemporarily secured to the head stock spindle. The coil winder'srotation direction switch is moved to the Clock Wise (CW) position. Theoperating lever is lowered and the speed control is increased graduallyto 60%. The core wire tension is maintained at 3-5 pounds-force.

Coiling is continued until the lead angle guide is within 1 foot,axially, of the tail stock chuck. The coiling process is halted byraising the operating lever and reducing the speed control to 0%. Thecoils are bonded to the core wire at the head and tail stock locationsover a 0.5 inch bond length. The three strands used to form the coil 78are cut at the core wire 60 and again care is taken to prevent damage tothe core wire 60. The spool carriage is returned to the head stocklocation.

Exiting the coil winding computer control program is accomplished bypressing the escape key (esc) at the computer keyboard, lowering theoperating lever, and gradually raising the speed control above 0%. Thissequence will create a user prompt to continue or exit to the main menu.A "M" is keyed to return the user to the main menu.

The completed drive cable 28 is removed from the coil winder. Theremaining coil strands at the head stock are removed by trimming.

Utilizing the above described method, a preferred embodiment of thedrive cable 28 is provided having an impedance of 50 ohms, a lowelectrical signal loss of 10-12%, and high shield and signalconductivity at high frequencies in the range of 10-50 MHz (whichincludes the preferred operating frequency of 30 Mhz). A cableconstructed according to the above described method can possess arelatively low loss, from 0.9 to 1.4 Db loss over the required frequencyrange. In the preferred embodiment, the drive cable 28 has a diameter of0.028 inch which is suitable for use inside a lumen of the elongatemember 26 having an internal diameter of approximately 0.035 inches.

IV. THE SHEATH

As mentioned above, during operation of the intravascular imaging system20, the drive cable 28 and sensor assembly 24 rotate at an angular speedwhile the transducer sensor 42 is excited and monitored. In order toaccommodate this rotation in the human body, the drive cable 28 andsensor assembly 24 are located in the flexible elongate member 26. Theelongate member 26 is composed of a non-rotating, bio-compatible sheaththat not only encloses both the drive cable 28 and sensor assembly 24but also serves to position the transducer sensor 42 at a desiredlocation in the coronary vasculature. Referring to FIG. 6, in thepreferred embodiment, the elongate member 26 comprises a tubular sheath80 having a distal portion 82 that can be positioned in a coronaryartery and a proximal portion 84 that extends out of the body of thepatient. The proximal portion 84 of the sheath 80 is fixed to astationary (non-rotating) component, specifically to a catheter manifold85 which in turn is connected to the housing of the uncoupling member 30(as described below and as depicted in FIG. 14). As shown in FIGS. 1, 2,and 4, the sensor assembly 24 is located in a lumen of the elongatemember 26 and specifically in a lumen 86 of the sheath 80 in a distalportion thereof.

In order to permit the transmission of the ultrasonic signal from thetransducer sensor 42 which is inside of the sheath 80 into the body ofthe patient (and the reflections back again), the sheath 80, or at leasta distal portion thereof, is made of a material that is transparent tothe ultrasonic signal. In the present embodiment, the sheath 80 or thedistal portion thereof is made of a TPX material, specifically amethylpentene copolymer plastic. The TPX material has an acousticimpedance close to water, a low coefficient of friction, and goodmechanical properties. Because the acoustic impedance of the TPXmaterial is close to water, very minimal signal reflections are createdat the sheath/blood interface. This characteristic allows the TPXmaterial to appear transparent to the transducer.

In a most preferred embodiment, the sheath 80 is formed of apolyurethane material. In order to make the sheath transparent to thepassage of ultrasonic waves, the transducer sensor 43 is mounted in thehousing 40 at a slight forward tilting angle, e.g. 10 degrees. Thisallows for the passage of the ultrasonic waves through the sheath 80without reflections.

The sheath 80 is formed having a low profile suitable for positioning inthe coronary vasculature. In a preferred embodiment, the sheath has 80an external diameter of 0.040 inch. The TPX material lends itself easilyto the extrusion process and can be readily drawn to very thin walldiameters. In this embodiment, the wall diameter of the sheath is 0.0025and the diameter of inner lumen is 0.035.

In addition to providing a non-rotating interface to the body, thesheath 80 furnishes other features. Because the TPX material has a lowcoefficient of friction, it provides a low frictional resistance bearingsurface between the internal drive cable 28 and the wall of the lumen 86of the sheath 80.

In addition, the sheath 80 provides mechanical support to the drivecable 28 in order to develop good "pushability" for cable manipulation.The TPX material possesses good mechanical properties for an extrudedcopolymer. The mechanical strength of the TPX material coupled with theaxial stiffness of the drive cable 28 generates a sufficient degree of"pushability", i.e. structural support in the sheath assembly, forpositioning the sensor assembly 24 in coronary arteries.

Located in the lumen 86 of the sheath 80 near the distal end is an innerlumen seal 87. This inner lumen seal 87 serves to establish a barrierbetween the interior of the sheath 80 and the patient's blood vessel.This shields the blood vessel from the turbulence caused by the rotationof the drive cable 28 and sensor assembly 24. When the sensor assembly24 is positioned in the sheath 80, the distal end of the sensor assembly24 is approximately 0.050 inches from the inner lumen seal 87.

At a distal end of the sheath 80 is a guiding tip 88. The guiding tip 88may be located in the lumen 86 of the sheath 80 distally from the innerlumen seal 87. The guiding tip 88 may be comprised of a radiopaquematerial, such as a coil of thin platinum wire. Platinum, with itsinherent radiopacity, wound in a coil configuration produces a soft,flexible, radio-opaque, crush resistant tip. Mounting the coil insidethe lumen 86 of the sheath 80 permits retaining the smooth outer surfaceof the sheath 80 thereby facilitating maneuvering the sheath 80 througha guiding catheter and eventually into a coronary artery.

As mentioned above, at a proximal end of the sheath 80 is located thecatheter manifold 85. The catheter manifold 85 has a first or main port89 generally aligned and communicating with the lumen 86 of the sheath80 and a second port 90 also communicating with the lumen 86 of thesheath 80. A strain relief coil 91 is located around the outside of theproximal end of the sheath 80 and extends into and is bonded between thesheath 80 and the catheter manifold 85. The catheter manifold 85 isutilized to connect the sheath 80 to the uncoupling member 30, asdescribed below. The drive cable 28 is installed into the sheath 80 viathe main port 89. The second port 90 may be used for flushing of thesheath 80, as described below.

The sheath 80 may also provide for a means of rotational compensation inorder to continuously calibrate the transducer's angular orientationduring operation. One of the drawbacks associated with rotatingultrasonic imaging devices is angular distortion between the encoders atthe proximal end and the sensor at the distal tip of the catheter. Thereare two main types of the distortions, those changing in time and thosefixed to the phase of revolution. The fixed distortion is caused byfriction or stiffness that causes a repeatable torque variation witheach cycle. This can be found in almost every rotating element to somedegree. The distortion that changes in time causes the image to rotateperiodically. The major source of this is the heart moving, which flexesthe elongate member causing a frictional torque variation synchronouswith the heart beat.

The present embodiment provides a solution to this problem by means ofan acoustical indexer. An acoustical indexer is a locational markingthat is put on, or built into the sheath to provide a rotationalregistration. This registration is constructed in the manner so that itcan be readily identified in the signal processing.

Referring to FIGS. 7a and 7b, rotational compensation markers 92 can beincorporated circumferentially in the wall of sheath 80 in a distalportion thereof. The markers 92 may be splines or patterns incorporatedon the interior surface of the sheath 80, as depicted in FIG. 7a, or onthe exterior surface of the sheath, as depicted in FIG. 7b. Preferably,the markers 92 are located at periodic positions 45 degree from eachother around the circumference of the sheath wall. The markers 92 can bemade from a variable thickness in the sheath material, but could be madefrom two different materials. These markers 92 may be formed in theextruding process for the sheath and may be made just in the region ofthe sensor or may extend over the entire length of the sheath. Each wallthickness change may be recognized by the signal processing unit 34 andcan be used to verify the transducer's angular position duringoperation. The thickness steps could be made at various ramp rates. Witha pattern in the sheath wall, the signal processing unit can follow theimage variation in distance. By following and holding steady one edge orfeature, the time variable distortion is corrected. This compensationability removes any discrepancy in a image due to an angular speedchange of the transducer.

By using a pattern of acoustic indexing markers as shown in FIGS. 7a or7b where the thickness is varied every 45 degrees, fixed distortions canbe corrected. Periodically, the data of the image representing thesheath would be analyzed to determine the correct time spacing of thetriggers. This data is transferred to the pulser that has a variabletime spaced pulse capability. By using a 1000 pulse per revolutionencoder connected to the motor that provides the synchronizing of themotor to the pulser, there is more than enough resolution to generatethe required pattern. The screen is divided up into 200 pie shapedangular divisions, each of these divisions is called a vector. For a 200vector screen, the pulser needs to generate 200 pulses per revolution bydividing up the 1000 pulses into the required spacings.

A block diagram of the indexing data processing is shown in FIGS. 8a and8b. A real time configuration tracks an edge in real time and adjuststhe pulse pattern very quickly, as represented in FIG. 8a. The data isintercepted from the raw data pipeline, processed and transferred to thepulser computer. The EKG signal would be useful for calibrating theimage to the heartbeat and removing the time-motion effect. A non-realtime configuration could be used almost as effectively, as representedin FIG. 8b. Here the data is processed and transferred periodically asneeded. The data is captured and processed by the main processor and theresult sent to a pulser computer that would pulse the excitation at theproper times.

A variation of this method but yielding basically the same result wouldapply a pulse to the sensor every increment of a motor encoder anddetermine the position of each vector in the pipeline processing.

Manufacturing of the Elongate Member

A sheath 80, as described above, may be made by first bonding a tubularportion into the catheter manifold 85 using an epoxy or other suitableadhesive. The sheath 80 should extend to a distal side of the entranceof the flush ports into the manifold 85. Care should be taken to ensurethat adhesive does not flow into the lumen 86 of the sheath. Then thestrain relief coil 91 may be installed into the manifold hub. The hub isthen filled with adhesive. This assembly is then allowed to cure.

Using an adhesive applicator syringe with a 0.025 inch maximum diametertip, the adhesive lumen seal 87 is installed in the distal end of thesheath 80. The seal 87 is preferably located 0.5 inch from distal tip ofsheath 80. The seal 87 should be 0.100 inch in total length. Next, thedistal marker coil is installed. Using a syringe, adhesive is applied to0.05 inch of the distal end of the marker prior to installation. Thedistal marker is installed in the distal end of sheath 80. The markercoil is allowed to interfere with the sheath's seal 87 by 0.05 inch.Then the assembly is allowed to cure at 140° F. for 4 hours.

Flushing Methods

The sheath 80 includes a means for flushing the sensor assembly 24 andsheath lumen 86. Any presence of entrapped gas or contaminants in aroundthe sensor assembly 24 reduces the performance of the imaging system.Any gas or contaminants on the surface of the transducer sensor 42 maygenerate severe reflections and essentially blind the transducer in thatregion. The flushing process assures that all gas and contaminants areremoved.

Flushing of the sensor assembly 24 and the sheath 80 may be provided bythree alternative systems:

Referring to FIG. 9, a first embodiment of the flushing system utilizesa flushing lumen 93 which may be a flexible tubular member having adiameter less than the diameter of the lumen 86 of the sheath 80. Theflushing lumen 93 may be fed through the catheter manifold's second port90 proximal to the distal seal 87 of the lumen 86. The flushing lumen 93is then pressurized with a flushing medium. The flushing lumen 93 isslowly withdrawn from the sheath 80 while pressure is maintained on theflushing medium. The process is continued until the flushing mediumflows from the proximal end of the manifold's main port 89 and theflushing lumen is removed.

Referring to FIG. 10 and 11, a second embodiment of the flushing systemis depicted. The second flushing embodiment uses a sheath 94 having duallumens, a main lumen 95 and an outer lumen 96. The outer lumen 96provides a flushing channel to the distal end of the sheath 94 where itcommunicates with the distal end of the main lumen 94 through a opening97 between the lumens 95 and 96. A flushing medium, typically water, iscontinuously fed under pressure through a proximal catheter manifold'sflush port 98 through the flushing lumen 96 from the proximal end to thedistal end, through the opening 97 into the main lumen 95, and backthrough the main lumen 95 from the distal end to the proximal end untilthe medium flows from the manifold's main port.

Referring to FIG. 12, a third embodiment of the flushing system isdepicted. This embodiment includes a sheath 99 having an air permeableseal 100 in the sheath's distal tip to allow entrapped gases to diffuseout during flushing pressurization. The seal 100 has a permeabilitywhich allows the air mass in the sheath lumen to be dissipated throughthe distal tip area in a reasonably short amount of time. In acomplimentary fashion, the seal's porosity is low enough to restrictwater mass transfer, i.e. the surface tension of the water coupled tothe porosity of the seal prohibits mass transfer. The seal may be madewith materials with permeabilities in the range of: 2 to 2,000,000ng/(s-m-Pa). This permeability range covers both flushing pressurevariations of 6.895 kPa to 689.5 kPa and flushing times of 1 second to1200 seconds. In the preferred embodiment, permeability for sheathflushing in 20 seconds at a 202.7 kPa flushing pressure through a 2.54mm long seal is 1290.1 ng/(s-m-Pa).

IV. COUPLING AND UNCOUPLING

Referring to FIG. 13, the elongate member 26 (with the distal drivecable inside thereof) is connected at its proximal end to the couplingmember 30 by means of the manifold 85. The coupling (and uncoupling)member 30 connects the distal drive cable 28 to the proximal drive cable32 which in turn connects to the proximally located components, i.e. thesignal processing unit 34 and the motor 36. By means of the couplingmember 30, the imaging system 20 provides a means of coupling anduncoupling the distal transducer side of the system from the proximalcomponents at a point outside the body of the patient.

This coupling member 30 provides several advantages for the imagingsystem 20. Providing for coupling and uncoupling of the distal sensorfacilitates loading and handling of the drive cable 28 and the sensorassembly 24 into the elongate member 26. Also, by providing a means forcoupling and uncoupling, the imaging system 20 can utilize larger size,less expensive components for electrical and mechanical transmissionproximally from the coupling location where the dimensions of thecomponents are not limited by the constraints of positioning in thecoronary vasculature. Thus, critical electrical information can betransferred from the rotating drive cable 28 to a less expensive,commercially available, stationary, 50 ohm coaxial cable whilemaintaining a mechanical link between the motor 36 and the sensorassembly 24. As required for the rest of components used for electricaltransmission, transfer of electrical information by the coupling member30 is preferably maintained in a controlled impedance environmentmatched to the transducer.

At the coupling location, the transmission of mechanical torque can alsobe transferred proximally to larger, commercially available componentsthat are less expensive to manufacture. Further, at the point ofcoupling between the proximal drive cable 32 and the distal drive cable28, a mechanical `fuse` may be provided to prevent torsional overload tothe drive cable in the body.

In the coupling member 30, the electrical and mechanical functions whichare united in the same components in the distal drive cable 28, aresplit into separate, adjacent cables one for the mechanical transmissionand another for the electrical transmission inside the proximal drivecable 32. Thus, in the uncoupling member 30, the electrical signaltransmission, which in the distal drive cable 28 is conducted by thecore wire that is rotating at operating speed, is transferred to anon-rotating coaxial cable connected to the proximal signal processingunit 34.

The coupling member 30 may be located approximately 60 inches proximalof the sensor assembly 24 so that it is outside of the patient's body.The coupling member 30 is comprised of a sleeve 101 inside of which iscontained a matable coaxial connector pair. The coupling member 30 inthis embodiment of the imaging system is provided by two assemblies thatare mechanically coupled together: a transducer pin assembly 102 thatconnects to the components on the distal side of the system, such as thesensor assembly 24 and the elongate member 26, and a slip ring assembly104 that connects to the components on the proximal end of the system,such as the signal processing unit 34 and the motor 36.

The coupling member sleeve 101 is formed by a first or distal sleeveportion 106 that is part of the transducer pin assembly 102 and a secondor proximal sleeve portion 108 that is part of the slip ring assembly104. These sleeve portions 106 and 108 may be made of a metal, such asaluminum. The sleeve portions 106 and 108 are held together through theuse of a coupling nut 110. Accordingly, the coupling nut 110 providesthe means for securing the distally located transducer pin assembly 102to the proximally located slip ring assembly 104 and their respectivecoaxial connector halves located therewithin together, as describedbelow, during system operation. This nut 110 may be removed or tightenedto disconnect or connect the distal components from the proximallylocated components.

Referring to FIGS. 14 and 15, the coupling nut 110 slidably fits overthe sleeve portion 106 and abuts against a shoulder 112 on the proximalend of the sleeve portion 106. The coupling nut 110 has threads 114internal thereto oriented in a proximal direction to engagecorresponding external threads 115 on the exterior of the slip ringsleeve portion 108, as shown in FIG. 15.

In each of the sleeve portions 106 and 108, there is provided one halfof the matable coaxial connector pair. As shown in FIGS. 14 and 15, amale component 116 of the matable coaxial connector pair is located inthe transducer pin assembly 102 and a female component 117 of thematable coaxial connector pair is located in the slip ring assembly 104.This coaxial connector mated pair 116 and 117 provides for both theelectrical and mechanical separation point between the transducer pinassembly 102 and slip ring assembly 104. Mechanical coupling between themated connector halves is controlled by the spring force exerted byinterference between the male coaxial connector shield spring contacts118 and the female coaxial connector shell 119. This spring forcegenerated between the male and female components of the matable coaxialpain allows for torque to be transmitted across the coupling member 30.This matable coaxial pair may be a commercially available coaxialconnector pair, such as made by Amphenol Corp., modified to be connectedin the coupling member 30.

This internal coaxial connection is made with a controlled impedancematched to the drive cable 28 and the transducer sensor 42, i.e. with animpedance of 50 ohms. Matching the impedance in the coupling member 30with these components avoids mismatch signal reflections, as describedabove.

Referring to FIG. 14, the transducer pin assembly 102 is connected tothe proximal end of the drive cable 28 so as to allow rotation of thedrive cable 28 inside the transducer pin assembly 102. Located at andcovering the distal end of the sleeve portion 106 of the transducer pinassembly 102 is a sleeve cap 120 having a passage 121 therethrough. Thesleeve cap 120 is secured to the sleeve portion 106 by stamping or acompression fit or other means. The sleeve cap 120 includes a nippleportion 122 which extends distally and in which is located the distalportion of the passage 121. The nipple portion 122 is connected to ormay be formed of the sleeve cap 120. Threads 128 located on the exteriorof the nipple 122 engage internal threads located in the proximal end ofthe manifold 85. A compression O-ring 129 may be provided between thedistal end of the nipple 122 and the manifold 85 to ensure a secure fit.

Located inside and connected to the sleeve 106 proximally from the endcap 120 is a coaxial connector bearing 130 and a bearing retaining ring131. The bearing 130 and end cap 120 define an interior portion 132 ofthe transducer pin assembly 102. The bearing 130 may be made of bronzeand oil-impregnated to provide for free rotation inside the pinassembly's outer shell. A drive cable clamp 134 is secured to the drivecable 28 so as to be located in the interior portion 132 of thetransducer pin assembly 102. The clamp 134 may be secured to the drivecable 28 by an adhesive or other means. A strain relief sleeve 136 maybe connected to or formed on the distal surface of the clamp 134 andextend distally on the drive cable 28 to a location through the nipple122 (e.g. 0.75 inches). The strain relief sleeve 136 may be made ofteflon.

Located around the clamp 134 is a shell 138. The shell 138 is comprisedof a first shell half 139 and a second shell half 140 that can besecured together such as by means of threads. When the shell halves 139and 140 are secured together, they also secure by compression the clamp134 between them. The shell 138 include a distal opening 141 and aproximal opening 142, both aligned with the passage 121 so as to receivethe drive cable 28. The opening 141 may also receive a portion of thestrain relief sleeve 136. A bushing 143 may be located in the proximalopening 142. The bushing 143 may be made of teflon. Connected to thedistal side of the shell 138 is the male portion 116 of the coaxialconnector pair.

The drive cable 28 is thus rotatably secured within the transducer pinassembly 102. The drive cable 28, the clamp 134, the strain reliefsleeve 136, the shell halves 139 and 140, the bushing 143 and the malecoaxial connector 116 are rotatable.

Referring to FIG. 15, there is depicted the slip ring assembly 104 whichforms the proximal half of the coupling member 30. In the slip ringassembly 104, the electrical signal is transferred from rotatable distalcomponents to non-rotatable proximal components, i.e. the electricalsignal transmission which is carried by rotating components distally istransferred to non-rotating components proximally. In addition, in theslip ring assembly 104, the electrical signal, which is carried by thesame components that transmit the mechanical torque distally, is carriedproximally by components separate from those that transmit themechanical torque.

As described above, the slip ring assembly 104 includes the sleeveportion 108 the proximal end of which connects by means of the threads115 and the coupling nut 110 to the transducer pin sleeve portion 106 toform the non-rotating coupling member sleeve 101. A slip ring end cap158 connects to and covers the proximal end of the slip ring sleeve 108.The slip ring end cap 158 includes a first opening 160 aligned centrallytherein and a second opening 162 offset from the first opening 160.Located in and extending through the first opening 160 is a slip ringdrive shaft 164. A proximal bushing 166 is positioned in the firstopening 160 around the slip ring drive shaft 164. An outer slip ring 167and an inner slip ring 168 are connected to the distal end of the slipring drive shaft 164. The outer and inner slip rings 167 and 168 areconnected distally to a modified coaxial connector 170 which forms theproximal portion of female portion 117 of the mated connector pair. Aproximal bushing 171 is mounted in the proximal end of the sleeveportion 108 around the female portion 117 of the mated coaxial pair.

Through the second opening 162 in the slip ring end cap 158 extend leads172 and 174 from the proximal drive cable 32. Specifically, the lead 172connects to the signal conductor and the lead 174 connects to thereference plane conductor of a coaxial cable in the proximal drive cable32, as explained below. The distal end of the lead 172 connects to aninner brush ring 176 and the distal end of the lead 174 connects to anouter brush ring 178. The inner and outer brush rings 176 and 178 may bemade of brass and may be approximately 0.063 inch wide. An inner spring180 is located between the inner brush ring 176 and the end cap 158 andan outer spring 182 is located between the outer brush ring 178 and theend cap 158. The inner and outer springs 180 and 182 bias the inner andouter brush rings 176 and 178, respectively, in a distal direction awayfrom the end cap 158.

The inner brush ring 176 bears against a set of inner brushes 184 andthe outer brush ring 178 bears against a set of outer brushes 186. Thesetwo sets of brushes 184 and 186 are mounted coaxially to each other. Ina preferred embodiment, each set of brushes 184 and 186 includes threebrushes, (only two brushes of each set are shown in FIG. 15). Each brushis located at 120 degree intervals to the other two brushes in itsrespective set.

The inner set of brushes 184 and the outer set of brushes 186 areslidably held by a brush guide 188. The brush guide 188 is mounted intothe inside of the slip ring sleeve 108. The brush guide 188 is acylindrical plug having two sets of three slots each located at 120degrees from each other (i.e. for a total of six slots) therethrough forretaining the two sets 184 and 186 of three brushes each. The brushguide 188 also includes a large central opening 189 through which passesthe slip ring drive shaft 164.

Biased by the inner spring 180, the set of inner brushes 184 bearsagainst and rides on the inner ring 168. The inner ring 168 is used toconduct the signal and is attached to the internal conductor of thecoaxial connector 117. Biased by the outer spring 182, the set of outerbrushes 186 bears against and rides on the outer ring 167. The outerring 167 is used for connection to the reference plane signal and isattached to the reference plane conductor in the coaxial connector 117and/or the sleeve 108.

The brushes provide the path for transferring the electrical signalinformation between the stationary inner and outer brush rings 176 and178 and the rotating inner and outer slip rings 168 and 167. In apreferred embodiment, the brushes are made of silver graphite. Silvergraphite provides for a brush material that is highly conductive andself-lubricating.

Relatively large brass slip rings are utilized to increase theconductive contact area available between both the slip rings andcoaxial connector 117, and the slip ring and brushes. The use of largecontact areas reduces electrical resistance and signal loss through theslip ring assembly 104.

In the slip ring assembly 104, only the coaxial connector 117, the slipring drive shaft 164, and the slip rings 167 and 168 rotate duringoperation. The sets of brushes 184 and 186, brush rings 176 and 178,brush guide 188 and sleeve 108 all remain stationary during operation.

Mechanical Coupling

In addition to providing for electrical transmission, the slip ringassembly 104 also furnishes the mechanical torque transmission acrossthe coupling member 30. The springs 180 and 182 in the slip ringassembly 104 develop the friction force which supports the torsionalload created by the transducer drive cable 28. Mechanical couplingbetween the mated connector halves 104 and 106 is provided by the springforce generated by the interference between the male coaxial connectorshield contacts 116 and the female coaxial connector shell 117. Thisspring force creates a friction fit between the mated connector pair 116and 117 and allows torque to be transmitted across the coupling member30. In the preferred embodiment, the torque transmittance in the slipring assembly 104 is tuned by adjusting the spring force to provide amaximum torque of 3 inch-ounces before relative slippage betweenconnector halves 116 and 117 occurs. This provides for a mechanical`fuse` feature in the system that ensures torque transmittance to thedrive shaft assembly and not the pin assembly shell.

The coupling member 30, comprised of the transducer pin assembly 102 andthe slip ring assembly 104, is easy to use and eliminates or reducesobstructions in the area of the patient. This facilitates the placementand manipulation of the elongate member 26 and sensor assembly 24 in thecoronary vasculature of the patient without the burden of having bulkycomponents in close proximity to the patient. The assembled couplingmember 30 has a cylindrical shape of approximately 0.75 inch in diameterand approximately 4 inches in length. (The transducer pin assembly 102is approximately 0.75 inch in diameter and 1.75 inch in length.)

The fact that the slip ring assembly 104 uses controlled impedancecomponents for electrical transmission except for a portion of a lengthless than 0.5 inches. This feature provides for the reduction of signalreflections from impedance mismatches.

V. THE PROXIMAL DRIVE CABLE

The distal end of the proximal drive cable 32 connects to the proximalend of the slip ring sleeve portion 108, as shown in FIG. 15. The distalend of the proximal drive cable 32 includes a proximal cable sheath 190that connects to the slip ring assembly sleeve portion 108. The proximalcable sheath 190 may be formed of a section of heat shrink tubing.Provided in the interior of the cable sheath 190 are the drive shaft 192that connects proximally to the motor 36 and the proximal coaxial cable194 that connects proximally to the signal processing unit 34. The driveshaft 192 and the proximal coaxial cable 194 are adjacent to each otherwith the drive shaft 192 aligned approximately along a central axis ofthe sheath 190 and the coaxial cable 194 offset therefrom. Proximallyfrom the cable sheath 190, the drive shaft 192 and the proximal coaxialcable 194 are enclosed in a proximal cable covering 195.

The drive shaft 192 connects to the slip ring drive shaft 164 inside thesheath 190. This connection is made by means of a shaft coupler 196which may be a tubular member made of DELRIN®. As described above, theslip ring drive shaft 164 extends distally from its connection to thedrive shaft 192 into the slip ring assembly 104 through the opening 160in the slip ring assembly end cap 158. The drive shaft 192 is preferablylongitudinally flexible yet torsionally rigid so that it can rotatethrough operation of the motor and transmit this rotation to the slipring assembly on to the drive cable and transducer assembly 24. Thedrive shaft 192 may be a flexible cable made of high tensile strengthsteel or stainless steel. A commercially available flexible drive shaftmay be used, such as S. S. White Industrial Products, Inc. shaft #098-9.

Also inside the proximal cable sheath 190 is the distal end of theproximal coaxial cable 194. The distal end of the reference planeconductor 198 of the proximal coaxial cable 194 connects to the proximalend of the reference plane lead 174 and the distal end of the signalconductor 200 of the proximal coaxial cable 194 connects to the proximalend of the signal lead 172. The coaxial cable 194 is preferably flexibleand is stationary, i.e. it does not rotate with the drive shaft 192. Amatching capacitor 202 may be connected between the signal and referenceplane conductors 200 and 198 of the coaxial cable 194 for impedancematching purposes. (The matching capacitor 202 would normally have aheat shrunk cover, which is not shown in FIG. 15). The proximal coaxialcable 194 may be a commercially available 50 ohm coaxial cable, such asRG 178 B/N, available from Belden Corporation.

Referring to FIG. 16, in the proximal cable 32, the drive shaft 192 andthe proximal coaxial cable 194 extend proximally from the proximal cablesheath 190 adjacent to each other inside the proximal cable cover 195.The proximal coaxial cable 194 may be enclosed in an isolation shield206 that may be made of tin plated copper braid. The drive shaft 192 inenclosed in a non-rotating metallic sleeve 208. At a branching member210, the coaxial cable 194 and the drive shaft 192 separate. Thebranching member 210 may be made of a heat shrink tubing. From thebranching member 210, the coaxial cable 194 extends proximally inside acoaxial cable jacket 212 to coaxial connector 214 that can be fitted tothe signal processing unit 34. From the branching member 210, the driveshaft 192 extends proximally inside a drive shaft jacket 216 to acoupling connector 218 to provide for connection to the motor 36. Themotor may be a 40 watt DC rare earth motor, such as manufactured byMaxon Motor Co., Model No. RE035-071-39EAB200A.

V. THE PULSER AND SIGNAL PROCESSING OPERATION

The signal processing unit 34 includes a pulser which generates the highenergy pulses that are converted by the sensor into an acoustical wavethat is used for imaging. A full single cycle pulser is used since itgives twice the energy as a half cycle pulser for the same peak voltageand further it gives better settling. For isolation between the highvoltage circuitry and the elongate member, a transformer is used. Highfrequency transformers are easier to design for cyclic waveforms with noDC frequency component, if fast settling is important. A full singlecycle pulse of the sensor generates a return signal with almost noincrease in ringdown time as compared to an impulse. Any increase in thenumber of cycles beyond one increases the ringdown time almost directlyproportionally.

For a good image, signal quality is very important. This means highamplitude with ringdown quickly to a -40 dB level. A pulser technique isutilized that provides for a sharper pulse and better ringdown of thesignal. Prior pulsers implement a pulse shape of an integer number ofhalf cycles at a given frequency. A pulser that is capable of generatinga pseudo-random pulse would be able to excite the transducer and settlethe reflections out by the sequence of pulses at certain amplitudes andat the right time.

As mentioned above, the size and shape of the sensor window is directlyrelated to the quality of the ultrasonic image obtained. Ultrasonicimaging in two dimensions, (i.e. of a cross section of the arterialwall) is acoustically a three-dimensional problem. Referring again toFIGS. 2-4, the objective for a good imaging system is to have a thinsharp rotating beam over the distance of interest, e.g., in a direction,y, radial to the artery wall. However, the beam, of course, propagatesin all directions. The performance of the beam in the two lateraldirections from the radial direction is termed the acoustical optics ofthe sensor. In the two lateral directions x₁ and x₂ (i.e. the directionsperpendicular to the radial direction), the beam shape is a function ofthe distance from the sensor, sensor focus, physical shape and theoperating frequency. For an imaging device that makes circular scans ofthe arterial walls, the resolution in the radial direction is limited bythe number of cycles of propagation of the pulse waveform. This time ordistance is determined typically by the -40 dB amplitude points of thewaveform, as illustrated in FIG. 17.

Using a rectangular transducer sensor is one of the keys to making avery small intravascular ultrasound device for use deep in thecoronaries. There are some tradeoffs with respect to circular apertures,but at the very small sizes the best performance is obtained from arectangular aperture.

The beam shape is a function of the housing aperture size in therespective direction. This function is

    Z=A.sup.2 /L

where Z is the near field distance, A is the aperture size, and L is thewavelength. For the above 0.5×1.0 mm window with a 0.056 mm wavelength(as defined by the frequency and media speed of sound), the near fieldis 1.1 mm in the x directions and 4.5 mm in the y direction. Thesignificance of the near field is that, for an unfocused sensor, thebeam width is nearly the aperture width through the length of the nearfield. In the near field the beam is rapidly changing in all directions.This is from the constructive and destructive interference patterns. Inthe far field the beam is more uniform and diverges. The far fieldbehaves as though the source was a point source. For focused crystalsthe beam can be focused up to the limit of the near field. A focusedbeam is narrower in the focused region but diverges faster thanunfocused outside this region.

For intravascular ultrasound in the coronary region, it is sought toobtain images out to about 5 mm in radius. For a window havingdimensions such as described, an advantage of the rectangular shape isthat even though the energy is spreading in the x directions, the energyin the y direction remains relatively constant through the distance ofthe region of interest, as illustrated in FIGS. 18-21. In the xdirections, or lateral directions as used in this specification, thebeam size is quite usable to generate good intravascular imagesthroughout the radius of interest. For a circular aperture of this size,the intensity would decrease very rapidly since the beam is spreadinguniformly in all directions. The rectangular aperture has betterdistance vs. energy dropoff along with a larger surface area. Forapertures 0.5 mm and smaller the rectangular shape has somecharacteristics that are more desirable for apertures than circularshapes.

Calibrated Waveform Pulser

As mentioned above, for radial resolution the ringdown of the signal isvery important and it would be desirable to have a single cycle responseto a impulse excitation. Typically, the excitation that is used iseither a half cycle type impulse excitation or a integer number of sinewave cycles.

There are significant advantages to using an excitation that uses a mainpulse rather than a modified pulse waveform to cause a faster -40 Dbringdown time. There are two major reasons for this. From computermodeling of the transducer, the resultant objective of the iterativeoptimization program is to generate a system transfer function that hasa smooth phase and magnitude over the widest frequency range. This isachieved by optimizing the value of peak pulse amplitude squared dividedby the integral of time weighted magnitude after the peak. By using anon-impulse excitation, the Fourier transform of the excitation isdifferent so that the frequency spectrum of the excitation is differentthan an impulse. The ideal impulse has constant magnitude frequencycomponents. By allowing the computer to vary the waveform from onediscrete time increment to the next, an optimum excitation waveform canbe generated.

There are limitations of the computer model, such as non-infinitebacking distance, surface irregularities, mechanical tolerances,impedance mismatches, etc. These variables result in the performance ofthe actual device to depart from what the model predicts. By usingbasically the same technique to calibrate a device, certain reflectionsand imperfections can be countered by using an optimized excitation.

This circuit could be implemented using a high speed Digital to Analog(D/A) converter, where the output could be programmed by a computer to apredetermined wave form, (see FIG. 22). This output could be amplifiedto any required level that is required. The optimized waveform isgenerated over a few hundred nanoseconds and is settled out before theimage data is received.

VI. ADDITIONAL PREFERRED EMBODIMENTS A. Sensor Constructions

Referring to FIG. 23, there is depicted an alternative embodiment forthe construction of the transducer sensor. In constructing a sensor, itis desirable to have a configuration that gives a uniform beam from onedevice to the next and is easy to produce. A uniform beam is necessaryfor both good repeatability of system performance as well as forimplementing other, more advanced data conditioning algorithms necessaryfor image enhancement.

Referring to FIG. 23, there is depicted an alternative embodiment of thetransducer sensor. As in the embodiment described above and illustratedin FIGS. 2-4, the transducer sensor in FIG. 23 is comprised of a severalseparate layers including a transducer core, conductive layers bonded toeither side thereof, a backing layer and a matching layer. In theembodiment shown in FIG. 23, a matching layer 301 (which may be composedof a PVDF material) is larger in dimension than sensor core 44 andincludes a overhang 303 on a proximal end. This overhang 303 allowselectrical contact between the conductive surface 45a over the sensorcore 44 and the center conductor of the coaxial drive cable (not shown).This provides for both a superior transducer surface with a very uniformactive area. This embodiment also significantly facilitatesmanufacturing. These features can be further enhanced through the use ofa conductive backing 305. The conductive backing 305 provides anelectrical contact between the sensor back surface and the sensorholder. The sensor holder is electrically connected to the drive cableouter conductor (not shown). The conductive backing can be composed of anumber of different materials, such as silver, tungsten, copper, gold ora number of other elements or alloys. The matching layer 301 can be madefrom PVDF, or other materials.

Other alternative embodiments include using a PVDF type material havinga conductive layer on both the front and back face of the sensor tocarry the signals to their connection. Behind the layer on the back sideof the sensor an attenuating layer may be needed to absorb the energycoming off in that direction. The two connections would be terminated atthe drive cable coaxial electrical connections. A further alternative isto extend the conductive faced matching layer and the conductive facedbacking layer from the sensor core to the proximal portion of the drivecable by integrating the flexible circuits into the construction of thedrive cable. In order to electrically insulate the two conductivesurfaces, an insulation layer is incorporated between the layers. Thiswould require no joints in the electrical sensor construction within thecatheter.

B. Imaging Guide Wire

An alternative embodiment of the present invention may combine thefunctions of a guide wire with those of an ultrasonic imager. A guidewire function is to navigate to a location of interest in a patient'svasculature and to position a catheter over the guide wire into placefor a procedure, such as balloon angioplasty. Because it would bedesirable to have a device that would image the artery before, duringand after such procedures, it would be advantageous to combine thefunctions of the guide wire and the imaging device. Most catheters areof a coaxial design so that once the catheter is in place the guide wirecould be withdrawn and an imaging guide wire put in its place.

Currently guide wires are used in dimensions of 0.018 inch or smaller.In the embodiment described above, the drive cable 28 has a diameter of0.026 inches and accommodates a transducer sensor having an active areaof approximately 0.020×0.040 inch. In order to combine the functions ofthe drive cable with those of a guide wire, the dimensions of the drivecable would be reduced in size to approximately 0.018 inch in diameter.The transducer sensor would be made with a housing aperture close to0.017 inch. At that size the image resolution would be substantially thesame as in the embodiment described above. With image enhancementtechniques described elsewhere in this specification, it would bepossible to have an image as good as or better than those currentlyachieved. A thinner transducer having a higher frequency or a differentmaterial could be used for the sensor.

A distal end of an imaging guide wire 350 is illustrated in FIG. 24. Adrive cable 352 can be constructed substantially as described above,except that for reducing the size from 0.026 inch to 0.018 inch, twocoils and a double braid would be used instead of three coils and aneight wire braid. This has the result of reducing the outer coils andconductors to 0.008 inch leaving 0.010 inch for a center conductor andinsulation.

In one aspect, the construction of the imaging guide wire would likelydepart from that of the embodiment described above and that is in themounting of the transducer sensor to the drive cable. In the imagingguide wire, the width of the active area of the sensor would be nearlyequal to the diameter of the drive cable. In all other respects, theconstruction of the transducer sensor portion of the imaging guide wirewould be very similar to that of the embodiment described above. Thisdrive cable 352 has a sensor holder 354 mounted at the distal endthereof. Unlike the sensor housing described above having oppositelylocated windows, the sensor mount 354 of this embodiment would notinclude a second window located oppositely from the transducer opening.Instead, the mount 354 would provide physical support under thetransducer sensor 356. Also, due to size constraints, there would belittle room for backing material on the back side of the transducersensor. This could be compensated for by several different methods. Forexample, the sensor could be made from a copolymer material which has alow acoustic impedance so that no matching layer would be needed tocouple to the fluid and further the impedance difference between thebacking support and the sensor material would be large enough so muchless energy would enter into the backing compared to PZT directlymounted. Alternatively, the energy that enters into the back support canbe somewhat dissipated and scattered by using a material such as aporous sintered type metal for the backing support and canceling outreflections with a calibrated pulse waveform. A major problem withcopolymer materials is the lower D33 coefficient. (D33 is the dielectricconstant in the thickness direction.) This makes a sensor of the samesurface area have a larger impedance. This impedance difference could becompensated for by using some of the techniques described elsewherewithin this specification or active circuitry could be placed next tothe sensor to buffer the signal to a lower impedance.

PZT materials can also be used in the imaging guide wire embodiment, butthey would likely need a front matching layer and a back decouplinglayer. The backing configuration may include a half wave decoupler whereits impedance is low with respect to both the sensor and backingsupport. This backing decoupler would work in the opposite manner fromthat of the matching layer, i.e. where a quarter wave length aids incoupling, a half wave length thickness helps in decoupling energytransfer between two impedances.

In the imaging guide wire, the electrical connections would be madethrough the backing support to the back side of the sensor and to theouter conductors of the drive cable. The front connection would be madethe same way as in the embodiment described above. For the copolymeralternative, connections would be made using one of the center drivecable wires and connecting the leads directly to the metallizedcopolymer surface using conductive epoxy or low temperature solder.

1. Imaging Guide Wire--Overall Construction

The imaging guide wire, as described herein, is an intravascular imagingdevice having an ultrasonic sensor located at a distal end of anintravascular wire sized and adapted to be located within the guide wirelumen of conventional catheters used for intravascular procedures. Assuch, the imaging guide wire has several significant advantages. Forexample, the imaging guide wire can utilize the path provided by theguide wire lumen of a conventional catheter to image at the arteriallocation to which the catheter is advanced. Moreover, in severalembodiments, the imaging guide wire may be provided with conventionalguide wire features, e.g. a floppy spring tip, to enable the imagingguide wire to be used as both a conventional guide wire for positioningan intravascular catheter as well as imaging features, e.g. a sensor, toenable imaging the intravascular regions accessible thereby.

In order to be utilized in the above described manner, an embodiment ofthe imaging guide wire 450 is provided, as shown in FIG. 34. The imagingguide wire 450 includes a tip section 452, a sensor section 454, a drivecable section 456, and a proximal connector section 458. As mentionedabove, an essential requirement for the imaging guide wire is that itpossess an outer profile of a size that allows it to fit through a guidewire lumen in conventional interventional catheters. In catheters thatuse 0.018 inch guide wires, the guide wire lumen has a diametertypically in a range between 0.020 and 0.022 inch. The diameter of theproximal section 458 of the imaging guide wire 450 may be as large as0.020 inches but the rest of the imaging guide wire should be not morethan approximately 0.018 inch. For use with catheters designed withguide wire lumens of other sizes, relative adjustments in dimensionapply.

2. Imaging Guide Wire Sensor

a. Image Resolution

The image resolution of the imaging guide wire is limited by the opticsof the aperture of the ultrasonic sensor. For an unfocused transducerthe resolution can be approximated by using the maximum between theangle of beam divergence and the aperture width. The formulaapproximating the resolution from angular beam spread is:

    x=R*Å/A

where,

x=resolution

R=range for sensor face

Å=wavelength of ultrasound

A=aperture width

For intravascular imaging, the depth of field where the best resolutionis desired is between 1 mm and 3 mm from the face of the transducer. Theouter limit of useful information is out around 4 mm to 5 mm from thetransducer face. With these constraints, a transducer should provide thebest performance in this range. For a flat, unfocused transducer, apreferred transducer aperture width can be determined for a selectedoperating frequency. The analysis in Table 1 is an approximation ofactual performance since beyond the near field the beam is uniform andapproaches this constant diffraction angle as distances increase. Thisanalysis is useful to get a coarse estimate of the expected resolutionas a function of the independent variables.

Table 1 shows the resolution for apertures of 0.5 mm, 0.4 mm, and 0.35mm for operation at 30 MHz. (A 0.5 mm aperture is disclosed in the firstembodiment described above in which the overall device profile is on theorder 3 Fr). The data of Table 1 indicate that for a system that willimage out to approximately 5 mm radius (the range necessary for coronaryarteries, for example), the optics limit the aperture to about 0.35 mm(0.014 inch). It should be noted that the resolution is improved out tothe radius of 4 mm by up to 30%.

    ______________________________________                                               A = 0.5 mm                                                                              A = 0.4 mm  A = 0.35 mm                                             x(mm)     x(mm)       x(mm)                                            ______________________________________                                        R = 1 mm 0.5         0.4         0.35                                         R = 2 mm 0.5         0.4         0.35                                         R = 3 mm 0.5         0.4         0.43                                         R = 4 mm 0.5         0.5         0.57                                         R = 5 mm 0.5         0.63        0.71                                         R = 6 mm 0.6         0.75        0.86                                         R = 7 mm 0.7         0.88        1.0                                          ______________________________________                                    

By increasing the frequency up to 40 MHz and utilizing a method forreducing the signal scatter from blood (as disclosed below), theresolution can be further increased in the area close to the sensorface. Moreover, the aperture size can be reduced. Table 2 shows theresolution for apertures of 0.5 mm, 0.4 mm, and 0.35 mm for operation at40 MHz.

    ______________________________________                                                                             A =                                      A = 0.5 mm     A = 0.4 mm A = 0.35 m 0.3 mm                                   x(mm)          x(mm)      x(mm)      x(mm)                                    ______________________________________                                        R = 1 mm                                                                              0.5        0.4        0.35     0.3                                    R = 2 mm                                                                              0.5        0.4        0.35     0.3                                    R = 3 mm                                                                              0.5        0.4        0.35     0.36                                   R = 4 mm                                                                              0.5        0.4        0.41     0.48                                   R = 5 mm                                                                              0.5        0.47       0.51     0.6                                    R = 6 mm                                                                              0.5        0.56       0.61     0.71                                   R = 7 mm                                                                              0.5        0.66       0.71     0.83                                   ______________________________________                                    

Table 2 shows that for a system that will image to approximately a 5 mmradius, the optics limit the aperture to about 0.3 mm (0.012 inch). Itshould be noted that, compared to the 0.5 mm aperture, the resolution isimproved out to the radius of 4 mm by up to 40%. The embodiments of thepresent invention for imaging guide wires relate in scale to this size.

The significance of a 0.3 mm (0.012 inch) transducer aperture size isthat this allows the imaging guide wire to possess an 0.014 inch overalldevice profile. This allows an imaging guide wire to be used withconventional over-the-wire type catheters that use a conventional 0.014inch guide wire.

There are two significant factors to be considered in providing an 0.014imaging guide wire. These factors relate to signal scatter from bloodand transducer design.

b. Imaging. Guide Wire Transducer Design

It is essential to consider the design and performance of the transducersensor as the wavelength width to length ratio is established in therange consistent with the optics requirements set forth above. With atransducer of the size required for an imaging guide wire, it can bedifficult to properly match the impedance of the transducer sensor tothe drive cable with available materials and at the requiredfrequencies.

There are two well known methods to model transducer performance. Themodel used for thickness mode vibration is known as the KLM model(Krimholtz, Leedom, Matthaei). This model is useful for modelingthickness mode transducers that are substantially clamped in the otherdimensions. With a transducer of a size that can be used in an imagingguide wire, the width mode of oscillation and excitation is significant.This diminishes the accuracy of the KLM model when applied to a sensorused in an imaging guide. This also makes the operation of a sensor withthis construction more difficult to work with. A rectangular sensor canbe made so that only its width is a consideration, however, with acircular aperture all directions should be considered.

Along with width oscillation being a consideration, the energy couplingcoefficient (kt2) decreases significantly as the clamped construction iscompromised. The coupling coefficient effects the signal level andringdown performance so it is advantageous to provide a material ormechanical configuration that will give as high a kt2 value as possible.This consideration must be reconciled with the contrary considerationsfor aperture size.

The other model and method of constructing transducers is based upon"phased array" considerations. This model can be similar to the modelabove used with clamped thickness mode if a complex loading impedance isused. With phased arrays, the width to thickness ratio (G=W/T) of eachphase element is preferably within a range where G=0.1 to 2.0 forreasonable performance. A maximum value for kt2 is obtained within therange of G=0.5 to 0.8. Accordingly, a sensor comprised of severalseparate elements, similar to a phased array, can be advantageouslyutilized in an imaging guide wire.

Such a sensor 500 is illustrated in FIGS. 35 and 36. The sensor 500 issliced parallel to the longitudinal axis of the drive cable 352 (shownin FIG. 34) thereby forming discreet transducer elements 502. Tominimize the width resonance of the sensor 500, the impedance betweenthe elements 502 should be kept as low as possible.

The electrical excitation for the sliced sensor 502 is similar to thatof the sensor 42, described above, and unlike conventional phased arraytype transducers. In conventional phased array excitation devices, eachphased array element is excited (and read) separately from the otherelements and separate electrical leads are required for each element. Adisadvantage of such conventional phased array sensors is that thenumber of separate, discreet leads for each element occupies asignificant area thereby limiting the size to which the device can bereduced.

As shown in FIG. 35, in one embodiment, the elements or slices 502 areexcited across the thickness direction of the elements. Alternatively,the elements can be excited across the width of each element. In thisembodiment, the thickness of the transducer 500 is limited andconstrained. However, through use of the sliced transducer face, theeffective width of the transducer can be increased for the capacitancecalculation. This allows the transducer to be made with the overallphysical dimensions required for an imaging guide wire but with animpedance matched properly to the other system components, e.g. thedrive cable.

Alternative embodiments of the sliced transducer sensor are shown inFIGS. 37, 38 and 39 in which the sliced sensor 500 is adapted with acircular aperture. With a circular aperture, the slices can be formed asstraight lines thus forming rectangular elements (FIG. 37), circularconcentric slices forming circular elements (FIG. 38), or spiral slicesforming spiral elements (FIG. 39). A circular aperture can also beformed on a rectangular substrate by metalizing the areas required tomake the circle area active. Piezoelectric material that is notmetalized on both sides and electrically connected to the signal cablewould not be active and would not effectively form part of the acousticaperture. In any of the alternative embodiment geometries, the use of asliced transducer provides the capability to properly match theimpedance of the transducer to the rest of the system components. Thus,transducer size is not a limiting constraint at these dimensions.

c. Additional Matching and Backing Layer Embodiments

For a solid sensor in a larger imaging guide wire, it is preferred thatthe transducer is air backed and have double matching layers. Doublematching layers allow more energy to be transferred out the front of thetransducer and less out of the back. By correct selection of theimpedance and thickness of the matching layers, an air backed sensor canproduce a near ideal pulse. It is desirable to keep the energy out ofthe metal or composite sensor holder. This feature is obtained byreducing the contact area between the two surfaces.

For an embodiment in which a fluid is trapped or otherwise exposed tothe sensor, it is preferred to keep any fluid or material out of thespace between the surfaces of the sensor and the mount. These surfacescan be treated to increase the surface tension between the surroundingfluid and bottom surfaces of the sensor and the mount.

Another alternative embodiment for accomplishing this is illustrated inFIGS. 40 and 40a. In FIG. 40, mounting tabs 508 are located over thetransducer 356 to aid in mounting the transducer 356 in place. Aprotective sheath 510 is included to provide a non-traumatic outersurface. By having a smooth end section, a gap 511 is formed between thesensor face 512 and the outer sheath 510. This gap 511 is preferablyfilled with water. Alternatively, there are a number of materials thathave the acoustic impedance of water and could be used as substitutealternatives for filling of this gap, such as silicon oil, castor oil,and many other fluids. It is preferred that this material bebiocompatible should a rupture occur. Alternatively, the space 511 canbe filled with a solid material that has nearly the same acousticproperties as water. Preferable materials include TPX, low density PEand silicon rubbers. Even though silicon rubbers have high attenuationthey may be suitable.

A further alternative embodiment is illustrated in FIGS. 41 and 41a.Instead of a single, solid material over the sensor 356, an exponentialmatching layer 516 is provided and shaped into the circular form of theholder 354. The exponential matching layer 516 is preferably formed of aseries of layers in which the impedance follows an exponential mannerfrom one layer to another. This type of matching layer is capable ofproviding as near to ideal matching as can be realistically achievable.

A further alternative embodiment is illustrated in FIGS. 42 and 42a.Less ideal but still suitable matching can be provided by forming thesheath 510 in a shape having a surface 520 that fully or partially fillsthe space in front of the transducer 256 thereby additionally providingthe function of the matching layer of the previous embodiment. Thissheath 510 with the formed surface 520 may be shrunk down over thetransducer section 356 providing for a non-filled sensor face. Theactive area of the transducer 356 should be limited to the flat areaprovided by the formed sheath 520. A formed sheath provides matchingnearly as good as that of the exponential matching layers of theprevious embodiment and may be easier to construct.

Energy that enters the sensor mount 354 should be minimized. By reducingthe amount of energy that is coupled into the backing, there is acorresponding reduction in the amount of energy that can reflect backinto the sensor. The contact area between the backing and the backingsupport is therefore kept to a minimum. The energy can further bereduced by limiting the energy that enters the backing support. Asmentioned above, one method is to use a composite metal and rubber orepoxy. This metal is preferably sintered or powder in epoxy. Another wayto attenuate the energy that enters the backing layer or support is toadd a quarter wavelength spline structure 524 around the backing layersupport as shown in FIGS. 43 and 43a.

d. Wedge Transducer

Another alternative embodiment for the transducer design in an imagingguide wire is shown in FIG. 44. In this embodiment, the transducer is awedge geometry transducer 530. Wedge transducers have been used in manyindustries to provide a broadband signal while coupling to a lowacoustic impedance medium. The wedge transducer material has a highacoustic impedance so that the acoustic energy is more easily coupledinto the material. A very good material for the PZT sensor and waterinterface is brass. This provides for a broadband pulse with a shortringdown. The angle between the wedge material and the transducer facematerial 531 causes two waves to be formed. One wave travels out of thewedge through the transducer face material 531 to the blood and arterybeing imaged. The other wave reflects off the face, stays inside thewedge and is attenuated so the sensor can see the return reflectionsfrom the forward wave. This attenuation can be obtained using a numberof different techniques depending on the level of attenuation needed.

One readily provided and effective method to provide the necessaryattenuation is to make the backing material 532 off that side the sameimpedance as the wedge material. A tungsten and rubber epoxy mixture canbe used for this purpose. The mixture requires a large percentage oftungsten by weight to get the impedance high enough to match the wedgematerial. This mixture is highly attenuative and a thickness of only afew mills of material is sufficient for the needed attenuation. To addto the attenuation at the wedge backing B interface, a quarterwavelength grating surface 534 can be machined or etched into the wedgematerial. This grating surface 534 reduces reflection back into thewedge 530 sufficiently without the use of a backing material at thatlocation. Any additional artifacts can be reduced or eliminated throughthe use of the calibrated waveform pulser, described above, or bycanceling out the repetitive return signal electrically.

The wedge transducer geometry allows for making a transducer in a sizenecessary for use in an imaging guide wire or even smaller. The minimumsize of a transducer formed with the wedge geometry would be limited bythe optics of the aperture, as described above. The wedge geometryallows the use of nearly all the cross section diameter for acousticaperture because the beam is bent to a small angle from perpendicular tothe wedge front face. Another advantage of the wedge design is that itprovides a mechanical structure needed to support the sensor and theguide wire tip. In this case this structure is an integral acousticalpart of the transducer.

e. Techniques For the Reduction of the Scatter From Blood

As mentioned above, with an imaging guide wire the frequency ofoperation can be approximately 40 MHz. At the short wavelengthscorresponding to this operating frequency, it is preferred to provide ameans to account for scattering due to structures (e.g. particles) foundin blood. In ultrasound imaging at such frequencies, such scattering canobscure the difference between the blood and artery or disease. ARayleigh scattering analysis that assumes spherical bodies fairlyportrays the observed phenomena.

One means for addressing this concern is to use a vector averagingcircuit to filter out the fast moving blood scattering return signal.The spatial frequency of the artery information is limited to twice theangle of the beam. In practice, it works best to sample about 25% fasterthan the maximum frequency. Faster sampling rates do not provide anymore useful information about the artery. Multiple fast sampling in the30 microsecond to 100 microsecond range provides information that can beused to average out noise, random pulses, and fast-changing informationfrom the blood scattering. For broad band noise, the signal-to-noiseratio is increased by the square root of the sample number. For randompulses and the type of signal received from blood scattering, thereduction is proportional to the sample number. This would be less thanproportional for very dense return signals since some may overlap.

Another way to reduce the scattering signal from blood is to use adouble frequency transducer. FIG. 45 illustrates an embodiment utilizingsuch a transducer. The transducer 356 includes a first sensor 540 and asecond sensor 542 located one over the other. An additional layer may beneeded between these two sensors to separate and isolate them into tworelatively narrow bandwidth sensors. With this construction, bothsensors 540 and 542 are pulsed at the same time and the return signalsare frequency multiplexed into different frequency bands. Thesefrequency bands are separated with analog or digital filtering. It ispreferred to acquire both signals together and process them with adigital fourier analysis. This requires a significant processingapparatus for real time imaging. Alternatively, analog filtering toseparate the frequency bands and dual channel data acquisition may alsobe used for real time implementation. The data acquired is processed bydual data pipeline data acquisition front ends, as described above. Theresulting information is then combined by a data pipeline function thatwould process the low frequency information to find the blood/arteryboundary and then switch to the high frequency attenuation and signalintensity for determining the material composition.

This technique effectively provides the output of both a low frequencysensor and a high frequency sensor at the same time. The low frequencyinformation is useful for blood-to-artery separation and the highfrequency information is useful for high resolution artery imaging. Thelower frequency is preferably between 20 MHz and 30 MHz. This gives agood low blood scattering signal and good resolution of the artery edge.The upper frequency is preferably about twice the lower frequency, butthis ratio may be from 1.5 to 3.

An alternative embodiment also incorporating two sensors in an imagingguide wire enables both side-looking and forward-looking imaging. Inthis alternative embodiment, one frequency sensor is pointed sidewaysand the other frequency sensor is pointed forward. This is readilyincorporated in a 3 French imager, as described above, because thelarger size device can possess an open end housing formed of a hypo tubein which the sensor is mounted. A forward-looking imager has the abilityto provide information about where the imager is being pushed.

Multiple sensor devices may include more than two sensors and twofrequencies. As long as the sensors do not overlap in frequencybandwidth significantly, more than two sensors could be used. This wouldbe useful for 3D imaging where narrower bandwidth sensors are used andmore of them are available for close, cross-sectional views.

3. Guide Wire Tip

FIG. 46 shows an embodiment of the imaging guide wire in which the tipportion 452 incorporates features so that the imaging guide wire 450 canbe used for both imaging and for positioning. In FIG. 46, the tipsection 452 of the imaging guide wire 450 includes a floppy tip 554 witha strain relief 556 section connecting the floppy tip 554 to the sensorsection 454. The strain relief section 556 provides for a variablebending force between the floppy tip 554 and the relatively stiff sensorsection 454. This can be provided by a gradually increasing core wirediameter or by gradually increasing the size or the diameter of the coilover the core wire.

In an imaging guide wire possessing positioning functions combined withimaging functions, one of the potential concerns relates to theinclusion of a long, floppy tip conventionally used with guide wires forsteering in an artery. The concern is that the floppy tip may twist offor scuff up the inside of the artery during rotation of the wire duringimaging. An embodiment feature that addresses this concern isillustrated in FIG. 47. In this embodiment, a mechanism 558 isincorporated into the imaging guide wire 450 that allows the tip 452 tostay stationary with respect to the artery when the imaging guide wireis being rotated for imaging but locks the tip 452 to the wire when itis being used for steering during wire placement. This mechanism 558includes a means for providing a fluid pressure on a hydraulic piston560 in the guide wire. When the piston 560 is pressurized, the guidewire tip 452 is locked to the body of the imaging guide wire. When thepressure is balanced across the piston 560, the tip 452 will rotatefreely.

b 4. Imaging Guide Wire Drive Cable

The drive cable section 456 is specifically adapted to address themechanical and electrical requirements of the imaging guide wire.Mechanically, the drive cable 456 preferably possesses very good torqueresponse in order to be used for intravascular positioning and goodlongitudinal stiffness for pushability. The drive cable 456 should alsoexhibit low angular whipping during rotation. Further, the drive cablesection 456 should be very straight. Electrically, the drive cable 456is preferably capable of sending a signal from one end to the other withminimum loss. In order to properly match the sensor impedance, highimpedance in the drive cable 456 is preferred. The electrical impedanceof the drive cable 456 is preferably in the range of 20 to 100 ohms.

An embodiment of the drive cable 456 is illustrated in FIG. 48. Thedrive cable 456 includes a core wire 564, an insulation layer 566, ashield layer 568, and a coil layer 570. The core wire 564 may possessseveral alternative constructions. In one embodiment, the core wire 564is formed of a solid wire. Alternatively, the core wire may be formed ofmulti-strand copper or silver-plated copper wires. The latter embodimentprovides good electrical characteristics and allows the drive cable 456to be relatively floppy. However, a multi-strand construction may notprovide sufficient longitudinal stiffness. Therefore, the core wire maypreferably be formed of a material having a high modulus of elasticitythereby increasing the longitudinal stiffness. Materials like stainlesssteel, tungsten, and beryllium copper are preferred. Of these, tungstenis most preferred since it has the highest yield strength and thehighest conductivity.

To provide for low electrical loss in the core wire 564, a highconductivity material is applied to the outer surface of the core wire563. Preferred materials for applying to the outer surface of the corewire 563 include silver or copper. Silver is most preferred since it hasthe highest conductivity. These materials are easily plated to athickness suitable for good electrical transmission. At highfrequencies, electrical current stays close to the surface of aconductor and therefore a 0.001 inch of conductor plating over the corewire is sufficient. In a preferred embodiment, taking into account bothmechanical and electrical requirements, the ideal thickness of thecoating is less than 0.001 inch.

The insulation layer 566 in the imaging guide wire separates theconductive core layer 564 from the conductive shield layer 568. Forelectrical purposes, this layer 566 is nonconductive and preferably hasas low of an dielectric constant as possible. If a solid wire is usedfor the core wire 564, it is preferred that a means be incorporated intothe insulative layer 566 to restrict longitudinal motion between thecore wire 564 and the outer coil 568. If the insulative layer 564 ismade of Teflon, a direct bond may be difficult to make between thelayers. In this case, movement between the core wire 564 and the outerlayers can be restricted at the joint between the drive cable 456 andthe sensor housing 354. This is preferably accomplished by using anonconductive sleeve to bond between the core 564 and outer layers thatwill be connected to the sensor housing 354. This sleeve is made out ofglass ceramic or other hard, nonconducting material. To bond between thelayers along the length of the drive cable, holes are formed in theTeflon at various patterns to allow glue or other bonding material to beused to connect the layers together.

A material other than Teflon can be used for the insulation layer 566.Such other materials include glass strands or a solid extrusion ofglass, kynar strands, or a ceramic extrusion. The extrusions would forma solid, uniform layer over the core wire out to a given diameter. Thestrands would then be expoxied to form a composite layer much like afiber glass or other composite structure that uses fiber and binder togenerate a unique high strength material.

The shield layer 568 is located over the insulating layer 566 to make upthe outer layer of a coaxial signal cable. The shield 568 can be madefrom a braid of wires or a coil of wires. In a preferred embodiment,these wires are rectangular silver-plated copper wires. A single layerof coils may be used to provide the smallest diameter drive cable. A lowresistance shield layer provides for RF emission shielding andsusceptibility. Cable loss is a function of the core and shield totalresistance, and accordingly, it is desirable to provide the shield withas low resistance as possible. For this reason, it is preferred that abraid or double coil is used for the shield layer.

The outer coil layers 570 are needed for good torque transmission forperforming the functions of both the drive cable and guide wire. Theouter coil layers 570 are formed of copper or alternatively other metalslike stainless steel. In a proximal section of the outer coil layer 570,a binder is used to bind all the layers together over a length thereofso as to make that portion of the imaging guide wire straight and stiff.This proximal section is from the proximal connector of the imagingguide wire to a location corresponding the end of the guide catheterwith which the imaging guide wire would be used. This distance istypically 130 cm. This allows the distal section of the imaging guidewire to be relatively more flexible where it needs to go through tightbends.

Another alternative way to provide additional stiffness in a proximalsection of the imaging guide wire drive cable 456 is to provide anotherlayer of material over the metal coil outer layer 570 along a proximalsection. This additional layer may be formed of other-than-metal strandsof glass, kevlar or other high strength materials. The strands would beused in a coil or braid layer over the core cable 570. The strands couldthen be epoxied to form a composite layer much like a fiber glass orother composite structure that uses fiber and binder thereby resultingin a unique, high-strength material. As described above, this can be adual section composite in which one section is made out of one fiber andbinder and the other section the same or different fiber and binder or acombination thereof.

5. Imaging Guide Wire Proximal Section

Referring again to FIG. 34, a proximal section 458 of the imaging guidewire provides several functions. These functions include a connectionfor electrical contacts for signal transmission, torque transmissionduring imaging, torque and longitudinal motion during guide wireplacement and a connection to an extension wire. FIGS. 49, 50, and 51show alternative embodiments for the proximal section 458 of the imagingguide wire. In each of the three embodiments, the proximal section 458has approximately the same diameter as the shaft portion 456 althoughthe proximal section 458 could range in size from a little larger thanthe shaft portion 456 to much smaller. In each of these embodiments, theproximal section 458 provides for electrical connection. The electricalconnection may be a static contact or a dynamic slip surface in whichcase there is a slip ring. Torque drive from the extension wire isaccomplished by fitting over a smaller (round square or other shape)wire 571 as in FIG. 49, or by fitting the wire inside a (round, square,or other shape) hole 572 as in FIGS. 50 and 51. The electrical contactconnection 573 may be in an axially displaced configuration as shown inFIGS. 49 and 49. Alternatively, the contacts may be one inside the otheras shown in FIG. 51.

With these proximal contact configurations described above and shown inFIGS. 49 to 51, an extension wire 574 is plugged into the end of theimaging guide wire. The overall profile of the extension wire 574 andthe imaging guide wire 450 maintains a small diameter as illustrated inFIG. 52. The extension wire 574 is used for at least two purposes.First, the extension wire 574 allows an interventional catheter to bepushed into place over the imaging guide wire after the imaging guidewire has been positioned at the desired arterial location. Second, theextension wire 574 is used to clap on to for steering and pushing theimaging guide wire into place initially. Alternatively, a short toolcould also be used that plugs into the end of the imaging guide wirethat would allow steering and pushing the imaging guide wire to thedesired location in the arteries.

When used for imaging, the proximal end of the imaging guide wire plugsinto an interface drive device 576 that provides torque drive and aninterface to transfer the electrical signals from the imaging guide wireelectrical contacts. This connection is illustrated in FIG. 53. Thissection 576 includes a proximal slip ring section 578 that wouldseparate the rotating mechanical drive from the electrical signals onstationary hardware. This drive interface may incorporate any of thethree slip ring alternatives, contacting, capacitive, and magnetic, asdescribed above.

Embodiments of the proximal interface 576 are illustrated in FIGS. 54aand 54b. An external motion restrictor 580 is preferably used whenrotating the imaging guide wire 450 to reduce any tendency for the wireto whip around in the radial direction. This restrictor 580 allowsmovement between the proximal assembly and the catheter through whichthe imaging guide wire extends. The restrictor 580 preferably allows forthe placement of the catheter as well as movement of sensor within thecatheter by pulling the imaging guide wire back and forward whilemaintaining the catheter stationary. FIGS. 54a and 54b illustrate twoalternative embodiments for achieving this motion restriction. FIG. 54ashows an embodiment in which a bellows type device 582 surrounds aportion of the proximal end of the drive cable 456. FIG. 54b shows anembodiment in which a non-rotating close-fitting tube 584 is fittedinside the catheter. The use of a sterile contact plug 585 and a sterilesheath 586 allow for a convenient setup procedure, using inexpensive,disposable or easily sterilizable parts.

6. Imaging Guide Wire Ancillary Equipment

An imaging guide wire is a relatively fragile device and accordingly itis desirable to provide a means to maintain the wire's straightness. Forexample, during installation of any guide wire intravascularly, thereare numerous possible occasions for the wire to be inadvertently bentand damaged. For this reason, an imaging guide wire holder and deliverydevice 590 may be used, as illustrated in FIG. 55. For installing theimaging guide wire in a guide catheter 592 or any other catheter orsheath, the fittings 594 are connected between the catheter 592 andholder 590. An extension wire 574 (not shown) may be connected to theimaging guide wire 450 and then pushed to move the imaging guide wire450 into place just before exiting the catheter 592. At this point, thefitting 594 is released and the extension wire 574 is pushed through theholder 590 while holding the imaging guide wire 450 steady. At thispoint, a clamp can be placed on the extension wire 574 to push and steerthe imaging guide wire into place. An interventional catheter may alsobe put in place at this time. The extension wire 574 is then removed andthe proximal imaging assembly is snapped into place. Imaging would beginby rotating the wire 450 and moving it in and out.

7. Imaging Guide Wire Methods Of Use:

a. Method 1

In a first embodiment of operation, the imaging guide wire 450 can beused as the primary guide wire, i.e. to both position and to image. Theimaging guide wire 450, described above, can be used in this manner.According to this method, the imaging guide wire is routed into place asa conventional guide wire. At this point, prediagnosis imaging may beperformed if considered appropriate by the physician. Imaging may bedone at this time by rotating the imaging guide wire in place. In thisembodiment, the imaging guide wire possesses a nontraumatic tip and asmooth covering over the wire to reduce the possibility that the wiremay damage the artery or the blood. In an alternative method ofoperation, a sheath may be routed over the imaging guide wire before thewire is rotated for imaging. This would also reduce or prevent anytrauma in the artery at the location of the sheath. The tip of theimaging guide wire may extend distally outside this sheath or theimaging guide wire tip could be drawn back into the sheath beforerotating. It should be noted that imaging can be performed in real time,and therefore to minimize problems with the rotation of a bare wire, thespeed of wire rotation can be low, e.g. a fraction of a Hz or even donemanually. If the imaging guide wire were rotated manually, a constantdisplay can, at a minimum, provide information concerning distances.

Once the imaging guide wire is in place and an appropriate therapy isdetermined, a therapeutic catheter, e.g. a balloon dilation catheter,can be routed over the imaging guide wire to the desired arteriallocation. At this stage, the imaging guide wire can again be used toimage by rotating the wire. Images obtained at this stage show thearterial cross section where the sensor is located. The imaging guidewire can then be moved and operated at the location where the treatmentis performed. After the treatment, the imaging guide wire can be left inplace while the catheter is removed and a second catheter is put in itsplace for yet another treatment if considered appropriate. Otherwise,the imaging guide wire can be moved to other locations to repeat theprocedure, if necessary.

In a further alternative method of operation, once the catheter is inplace over the imaging guide wire, the catheter and the imaging guidewire can be moved together to another arterial location. Here, the tipcan extend distally outside the catheter and the image guide wire can berotated and pushed to facilitate advancing and positioning the wire andcatheter to the desired site. When used in this manner, the imagingguide wire can be used as a conventional guide wire. This has thepotential to save time since the catheter would not have to be pulledback and replaced immediately for imaging and treatment.

b. Method 2

The above described method of operation is directed to the use of theimaging guide wire in conjunction with a conventional over-the-wirecatheter in which the guide wire lumen extends the length of thecatheter. Other types of catheter designs are available, such as thetype of catheter in which the catheter has a short guide wire lumen(SGWL) at the distal end of the catheter and in which the guide wireoccupies a location adjacent to the catheter proximal of a proximalentrance to the short guide wire lumen. If the imaging guide wire isused with a catheter of this type, somewhat different steps of operationmay apply. With a short guide wire lumen catheter, the imaging guidewire may be first put in place in the artery. The imaging guide wire maybe provided in a somewhat longer length, or a short extension wire canbe connected to the device used in method 1. The short guide wire lumencatheter is then advanced over the imaging guide wire and pushed intothe desired arterial location while holding the proximal end of theimaging guide wire. A concern is that the imaging guide wire is adjacentthe short guide wire lumen catheter within the guide catheter over aconsiderable portion of its length. It may be desirable to reinforce theimaging guide wire shaft along this portion of its length so that it canbe rotated without whipping. This may be done by applying a stiffeningcomposite layer, for example. Alternatively, a reinforcing sheath may bepositioned over the imaging guide wire up to the proximal guide wirelumen entrance. Such a reinforcing sheath is described in copendingapplication Ser. No. 07/725,064 filed Jul. 5, 1991, the entiredisclosure of which is incorporated herein by reference. With theseadditional considerations accounted for, the imaging guide wire may beoperated in a manner similar to those set forth above.

c. Method 3

According to this method, a conventional guide wire is first used with aconventional intravascular catheter. The guide wire and catheter arepositioned in a conventional manner. The conventional guide wire is thenwithdrawn and the imaging guide wire is put in its place via the guidewire lumen of the conventional catheter. In this method, the imagingguide wire can be constructed somewhat differently from the imagingguide wire described above. If used with a separate, conventional guidewire for positioning, the imaging guide wire used in this embodimentneed not possess a distal steering tip. Instead, all that would berequired would be a smooth end sensor section. A non-traumatic shortsoft tip may be included at the end of the sensor section for extendingbeyond the end of the catheter into the artery. Further, in thisembodiment the proximal end of the imaging guide wire does not have topass through the catheter, and accordingly, there is no size restrictionon how large it is. A proximal connection could be used very similar towhat is described for use with the 3 Fr imaging device, described above.

d. Method 4

In this method, a conventional guide wire is used with a dual lumencatheter. A dual lumen catheter is disclosed in copending applicationsSer. Nos. 07/704,828, filed May 23, 1991 and 07/809,715 filed Dec. 18,1991 the entire disclosures of which are hereby incorporated byreference. According to this method, a conventional guide wire isadvanced into the desired arterial location. The dual lumen catheter isrouted over the conventional guide wire using one of the lumens. Thislumen can be the full length of the dual lumen catheter or can be mergedinto the first lumen at any point proximal from the distal tip. For thedual lumen catheter, the conventional guide wire is then partiallypulled back to allow the imaging guide wire to image through thatsection and extend beyond the end of the dual lumen catheter. In thismethod, any of the imaging guide wire embodiments may be used. If animaging guide wire, as described above in method 1 is used, the imagingguide wire could be left in place and the catheter could be moved orexchanged over it.

C. CCD Data Capture and Sensor Configurations

Among the major obstacles associated with ultrasonic imagingconfigurations are the matching of impedances between the sensor and thesignal cable, transmitting the signal down the cable with minimal loss,and maintaining a high signal to noise ratio. For phased array sensors,described below, and two dimensional sensor arrays, there are additionalproblems related to parallel signals, such as crosstalk and multiplexinglimits.

In a further embodiment of the present invention, an imaging transducersensor is provided having a charge coupled device, (CCD), associatedtherewith. The CCD is an integrated circuit that could be used tocapture the high frequency waveform of the sensor and send it back tothe proximal end of the device preferably both amplified and at a lowerfrequency. The charge coupled device (CCD), as referred to herein, maybe one of a family of charge transfer devices which may also includecharge injection devices.

Referring to FIG. 25, there is depicted a distal end of a imaging device360 including a CCD 362, a PZT transducer 364, a matching layer 366, abacking material 368 all mounted in a holder 370. The signal from thetransducer 368 is input to the CCD 362. The electrical connections 372between the CCD 362 and the signal and power wires 374 would be madeusing standard IC wire bonding techniques. The input impedance of thecell can vary widely based on the cell capacitance and input resistance.This input impedance would be designed to give the best pulse ringdown.The pulse is generated by the CCD IC or alternatively the pulse may comefrom a proximal pulser, as in the embodiment described above. After thepulse, the CCD would be clocked to store the input waveform from thesensor 364. After the waveform is acquired, further clocking of the CCDarray at a slower frequency will allow the "reading" of the stored valueand transmitting this to the proximal electronics for further processingand display.

The device 360 provides numerous advantages for intravascular ultrasoundimaging. It allows nearly perfect impedance matching independent of thesensor. It allows the reduction in frequency of the transmission of thesignal to the proximal end at very low noise susceptibility or emission.This would allow the reduction of the current coaxial wire design to asingle wire signal design. As few as two wires would be necessary if thepulsing is remote and communications are done over the power lines.

In the embodiment shown in FIG. 25, the CCD 362 and the sensor 364 arenext to each other. By using a PZT sensor with PVDF matching layer withan overhang tab for top contact connection, the connection between thesensor and the CCD is made by having a large metal pad on the IC tocontact the PVDF conductive layer.

In an alternative embodiment 375 shown in FIG. 26, a transducer 376located over a CCD 378. This embodiment uses a copolymer material forthe transducer 378 and mounts it over the CCD 378. This provides anelectrical sensor plane as part of the CCD 378 by using a large area topconductive layer.

In further related embodiments, a CCD array can be used in sensordevices having more than one sensing area, e.g. phased arrays and lineararrays such as described in the specification below, for sequentialsensors mounted along the axis of the device for 3-D imaging. Suchembodiments utilize the same type of circuit for the CCD array asdescribed above but use parallel paths. Such a configuration is similarto that currently being used in cameras. The CCD imaging catheterfunctions as follows. Photons excite the electrons that are stored intoa 2-D CCD shift array. Once the values are loaded into the shift array,they are then shifted to one edge of the IC one row at a time where theyare shifted in the other dimension to a circuit that measures, amplifiesand sends out the information one pixel at a time. A device very similarto this could be used in phase arrays, where like the single sensor CCD,the signal is read and input into the CCD at one end of the shiftregister and it comes out the other end. This would allow thesimultaneous acquisition of all of the sensor array elements and allowthe transfer of the total information to the proximal circuitry withvery little loss or distortion from noise or crosstalk. Here, as in thesingle sensor design, the sensor material could be located over the CCDor next to it.

This concept could be extended further in an embodiment of a CCDacoustical sound beam imager. This device would be similar to that ofCCD arrays used in cameras, however, instead of having a cell areadesigned to generate electrons from a light source, the charge couldcome from a small area of piezoelectric material. The piezoelectricmaterial could be placed over the CCD surface areas would be defined onthe top metallization layer of the IC that would capture and transferthe piezoelectric charge into the input cell of the 2-D shift registerarray. Once the data are loaded into the shift array, they are thenshifted to one edge of the IC one row at a time where they are shiftedin the other dimension to a circuit that measures, amplifies and sendsthe information out one pixel at a time. This device would be able totake a snapshot of all the acoustical 2-D wave front one point in time.

This concept could be even further extended to provide for a shiftregister for each of the acoustical pixels. This would allow forcapturing all of the 2-D waveforms in time. Such a device would be veryuseful for 3-D imaging in a non-moving device. A forward-lookingconfiguration could be constructed in which the device is placed at theend of the catheter or is placed behind an acoustical lens in the focalplane. This would allow the acquisition and direct display of the imagewithin the focal region of the device. Acoustical excitation could begenerated by a single pulse from a dispersive acoustical generator. Thisgenerator could be a piezoelectric layer over the CCD.

D. Sequential Sensor Mounting for 3-D

Three dimensional (3-D) images would be very useful to visualize theextent of certain diseases present in vessels. 3-D imaging allows forslicing, rotating and displaying the information so that volume andcross sections can be visualized. A 3-D reconstruction requiresinformation from a number of 2-D cross sections as well as informationabout their corresponding position along with the vessel. Acquiringinformation for 3-D reconstruction can be obtained by starting at oneposition in the artery and moving the sensor past the area to bereconstructed. There are drawbacks associated this technique, however,such as the fact that in coronary arteries the rotational axis of theartery is hard to define in time since this axis is moving. Also,obtaining good distance measurements along the length of the artery canbe difficult because of the stretching of the drive shaft or the sheathespecially if the whole catheter has to be moved. This stretching canpresent a problem since the displacement distance may be measuredproximally with a distance transducer. For example, as the catheter orthe drive shaft is pushed in from a proximal end, friction could preventthe sensor from moving at all. This would produce a significantdistortion in the 3-D reconstruction. Also, the duration of time neededto acquire all the information required for a 3-D reconstruction couldbe a drawback by limiting the capability for rapid update of the 3-Dimage.

Referring to FIG. 27, there is depicted a distal end of an ultrasonicimaging device 390 that provides for 3-D imaging. This device containsmultiple sensors 392 along its axis. The multiple sensors 392 arelocated and mounted in a mounting holder 394 which is mounted on adistal end of a drive cable 396. The holder 394 would be connected tothe drive cable 396, and driven thereby, in a manner similar to thatused for mounting a single sensor holder. The multiple sensors 392 mayinclude an arbitrary number of sensors depending on the number of crosssections required. Each sensor would be located at a constant, knownspacing in the mounting holder 394. The sensor holder 394 may haveflexible sections 398 between each of the sensors so that each of thesections can flex as it is being rotated. This could also facilitatedelivery and use of this device. Each of the multiple sensors 392 wouldbe operated to scan the cross section where it is positioned.

There are alternative transmission schemes for transmitting theinformation signals from each sensor section to the proximal end of thedevice. For example, the signals from all the sensor sections could betransmitted in parallel, or alternatively, signals from each individualsensor section could be transmitted one at a time by multiplexing, or acombination of these two methods could be used. A multiplexer wouldselect which sensor section is currently active and send its signal downthe cable. There may be some advantages in transmitting one signal at atime using a multiplexer at the proximal end of the sensor array, suchas a reduction in crosstalk between channels and the elimination ofmultiple high frequency signal wires.

The conditioning hardware for reconstructing a 3-D image in a reasonableamount of time may include parallel processing units each working on asection of the image. Each one of these could require a powerfulprocessor. Economies may be provided by using a network of Intel I860type processors. The data acquisition and data pipelining would be verysimilar to that described elsewhere in this specification. The 3-Dprocessing might be best implemented in the raw data pipeline.Alternatively, it could be implemented as a parallel data path into agraphics pipeline allowing simultaneous display of one of the sensor'scross section being displayed in real time along with a 3-D image of thetotal region.

In a further embodiment, these multiple sensors could be used in aphased sensor operation in which the beam is swept and pointed along theaxis of the device. This may be a desirable configuration since it wouldallow some "forward-looking" along with 3-D acquisition. If this wereimplemented, it would be preferable that the sensor elements beconstructed having a smaller dimension in the direction along the deviceaxis.

For a sensor array configuration, the sensor sections would not have tobe rotated to obtain an image. By holding the device motionless, animage would be obtained of a cross section of the wall of the arteryfacing the sensors. This would for most applications be very usefulinformation. 3-D information could still be obtained by rotating thewhole device.

E. Acoustical Indexing for 3-D

An alternative approach to 3-D imaging is shown in FIGS. 28 and 29. Thisalternative approach would use a longitudinal indexing pattern 400 on asheath 402 for 3-D imaging. The indexing pattern could be made to varyalong the length of the sheath 402. The pattern 400 would be used todetermine the location along the length of the sheath 402 at which thetransducer (which would be insider the sheath as in the previouslydescribed embodiments) is located. This information could be used foracquiring 3-D information of the artery as the transducer was moved withrespect to the sheath. The pattern 400 could be posess a binary pattern,a gray scale pattern, or other patterns to indicate a change in positionbetween the sheath and the transducer. The pattern could be applied overjust the distal length on the sheath or over the entire length.

The pattern 400 may be encoded for incremental or absolute registration.For incremental registration, only one bit of information would berequired. In such a case, external direction information would typicallybe generated. For absolute registration, two bits of information wouldbe provided and used in quadrature, thereby allowing the direction to bedetermined. For absolute position information, gray scale encoding maybe preferable. Gray scale coding has the property that only one bitchanges in going from one state to the next. This prevents errorscompared to binary scale for example, since there is no way of ensuringin binary scaling that all bits will change simultaneously at theboundary between two encoded values for binary or other codes.

Patterns for both radial acoustic indexing and 3-D lateral indexing maycoexist on the sheath. Both patterns could be formed of the sheathmaterial or could be formed of different materials. One pattern could beformed on the inner side of the sheath while the other on the outerside. Also, these patterns could be formed on the same surface.

F. Hydraulic Drive and Acoustic Indexing

Using acoustical rotational indexing allows determination of the sensorangular position independent of the proximal angular position of amechanical drive shaft or cable. With this capability, means other thana mechanical drive shaft can be used to scan the vessel with a rotatingacoustic beam. In a further embodiment of the present invention depictedin FIG. 30, there is provided a rotating imaging device 408 for scanningof a vessel of a patient with a rotating acoustic beam that is driven bymeans other than a mechanical drive cable. In the embodiment shown, arotatable mirror 410 is driven by a rotating hydraulic source 409. Therotating hydraulic source may be a jet and fin type turbine 412. Theturbine 412 would propel the mirror 410 in a rotational direction. Thespeed of rotation of the mirror 410 could be controlled by varying thefluid flow rate. Using fluid, the rotation of a mirror 410 would be verysmooth since there would be little friction from rotating shaftscompared to mechanical drive devices. Bearings 414 could be provided toprovide for smooth rotation. Feedback to the pulser for pulsing andspeed monitoring would be provided using an acoustical indexing pattern416 on the sheath in the rotational direction as described above. Atransducer sensor 418 could be mounted either distally or proximallyfrom the mirror and aimed to direct an acoustic pulse toward the angledface of the rotating mirror 410.

This configuration provides advantages for combining other functionsinto the device. With no moving parts over most of the length of thedevice, there is available substantial room to add other features. Forexample, it would be possible to integrate a balloon onto the device.The hydraulic course would already be present, and if the balloon isported to the same fluid used for driving the mirror, all that wouldneed to be done to inflate the balloon would be to control the inputpressure independent of the output flow rate. This could control boththe inflation pressure as well as the mirror rotation speed.

In a further embodiment, a rotating sensor 420 could be used instead ofa rotating mirror by using a slip ring holder 422 to couple the signalsto and from the rotating sensor 420 to a signal cable on a catheter, asshown in FIG. 31. A hydraulic turbine 424 would drive this device justas described in the embodiment above with the mirror. As in the previousembodiment, an acoustic encoding pattern 426 would be included on thesheath portion of the device. This embodiment has the advantage that thesensor 420 could be designed with a thin backing with a holetherethrough large enough for admitting a guide wire 428 through thecenter of the device. This would provide for over-the-wire placement ofthe device.

G. Data Graphics Pipeline Architecture

In ultrasonic intravascular imaging, a large amount of data needs to beprocessed between the transducer being pulsed and the image beingdisplayed and various means can be used for this processing. Forexample, processing can range from all analog to all digital. In mostdigital systems, the conditioned signal is acquired through dataacquisition, processed by a computer, and displayed through somegraphics hardware. This can be accomplished over a computer buss as longas there is a limited amount of transferring being done. Current systemsare very basic in the digital conditioning and image processing, and canutilize this approach.

It would be preferred to use digital conditioning functions to enhancethe ultrasonic image or to provide for feature extraction. This wouldlikely require a different data flow architecture to provide foradditional data transfers needed to produce the image reasonablyquickly. FIG. 32 depicts a pipeline structure that provides thisarchitecture. This architecture includes a dual pipeline: one for rawdata and another for graphics data. The analog input from thesensor/conditioning is acquired from a high speed data acquisitioncircuit. This circuit synchronizes the raw data pipeline and transfersthe data down the pipeline at a lower speed. The data is passed from onefunction to the next in real time or near real time speeds. Thispipeline basically processes polar data. Since there would be much lessdata in the polar domain, it would be preferable to process this data asmuch as possible. These processing functions may include deconvolutions,fourier transform processing, neurocomputing processing or othertechniques to enhance the raw data and do feature extraction.

At the end of the raw data pipeline, the data is converted to a graphicsdata stream through a large "look up table" (LUT). This LUT essentiallyperforms a polar to rectangular conversion. There are other ways togenerate the graphics data from the raw data, but this is the preferablemethod. The graphics data can then be handled in the graphics pipeline.Processing functions performed here are those that preferably should bedone in rectangular instead of polar coordinates. These may include edgedetection, area calculations/manipulations, logical pixel edgesmoothing, other area operations, and image overlays.

This architecture is ideal for intravascular imaging applications sincethe data can be acquired and processed from one function to the nextwith minimal time delay. The pipeline structure is very flexible forfeature enhancements and additions, all that needs to be done is tochange a cable between the appropriate location to add a new pipelinefunction. This structure can accommodate a variable number of pipelineelements, as needed.

A small variation on this architecture would include the addition ofparallel pipelines. This could be done for example by taking the rawdata acquisition output, branching off to a second LUT, and combiningthe two at the initial graphics pipeline function. This would allow twodisplays of the same raw data at the same time in different locations onthe screen. This would be desirable if a real time enhanced display isdesired while at the same time showing a slower 3-D reconstruction orenhanced feature detection.

The data pipeline and graphics pipeline architecture, as describedabove, are advantageously integrated into a system environment. FIG. 56shows the pipeline structure integrated into one type of systemenvironment. FIG. 56 shows how the communication portion of thearchitecture can be implemented to allow the central system CPU tohandle pipeline setup and configuration. This allows user input toeffect changes in overlays, images and signal conditioning of data. Notevery pipeline function may require a direct interface to a common buss.An alternative to common bussing is daisy-chained communications. Here,the common processor would be able to perform the setup andconfiguration tasks using a serial or parallel communication link. Anexternal controller may be provided in the overall system configuration.This controller may issue commands to the system or may be directlymemory-mapped to the functions on the system. This intersystemcommunication may employ techniques known and accepted to those of skillin the art. In the first method, the external controller may beconnected in a serial or parallel manner and communicate with the systemCPU. As with the keyboard, these commands can be queued and processed,or a handshaking can occur for synchronized command execution andcommunication. The memory-mapped external system control is performed byhaving the external system take control of the system common buss andaccessing the hardware and memory directly.

H. Acoustic Waveform Deconvolution

A major goal in acoustic imaging is high resolution of the image. It isdesired to have an image with features as well defined as possible. Oneof the limitations to the image resolution is the attenuation of thesignal with frequency. If there were a much higher signal to noiseratio, a higher frequency could be used, yielding a higher resolutionimage for a given size aperture device. Alternatively, a smaller devicecould be produced having the same resolution. Resolution may be definedas the distance at which two points are barely distinguishable. Withacoustic beams using traditional imaging techniques, the resolution ofthe image is a function of the beam width at the points of interest.

In ultrasound imaging this is complicated by the fact that coherentacoustic fields are being used, so at a certain distance apart tworeflectors can appear as one or two objects depending on theinterference phase. This interference pattern or "speckle" can give asense of higher resolution object separation than is actually possiblewith a beam of a given width. This speckle pattern can be useful becauseit gives a material a texture that can be meaningful for associatingcertain properties or identifying the material.

In the near field, an unfocused beam varies rapidly from point to pointradially from the sensor surface as well as laterally through the beam.Quantifying and defining resolution is difficult in the near field.Imaging in a more uniform beam can provide a more predictable result. Ina focused beam, there are two regions where the beam is somewhatpredictable. In the far field, the beam is of the form of an airy diskfor a circular aperture and a mathematical Sine² function for arectangular aperture.

For signals, convolution is the summing in time of a finite patterninput into a output pattern using a transfer function. Deconvolution isthe reverse process where from a given output the input pattern isfound. For devolution, the accuracy of the determined input is afunction of the accuracy of the measured output function and theaccuracy of the transfer function.

A transfer function also exists for acoustical imaging but is in twodimensional space and time as well. The two dimensional space transferfunction is proportional to the intensity of the acoustical beam at agiven radius. This problem is more difficult than the one above but thesame basic principles apply.

For acoustic beams, a knowledge of the beam shape and point intensity asa function of time is the major variable in performing a deconvolutionon the acquired information. The beam shape and point intensity valuesare a function of sensor aperture, surface, uniformity, sensorconstruction tolerances, and diffraction/reflection of where the beamhas come from and gone though. To know the beam values to any greatdetail is a very time consuming task if they are computed or measured.

In the near field and not in a focused region, the beam shape is varyingrapidly as distance from the sensor changes. In the focused region andin the far field, the beam is more uniform and predictable. In theseregions deconvolution will be of some use. In the other areas of thebeam as sensor technology produces more uniform sound beams, thistechnique will enhance the entire image. For the current system, most ofthe imaging is done in the far field in the rotationally lateraldirection.

The resulting benefits from this routine are a sharper apparentresolution and a higher signal to noise ratio. The side lobes magnitudeand the main beam size are the main determinates of the resolution ofthe image. Deconvolution will improve upon the limits set by both ofthese factors. The noise is reduced if the noise waveform has a smallsimilarity with the acoustic transfer function, which is mostly thecase.

The standard technique for performing a deconvolution is to use Fourieranalysis. This is done by taking the Fourier transform of the output,dividing this by the Fourier transfer function, taking the inverseFourier transform, and using the result. For a system where the transferfunction is varying with time and space, the exact procedure is morecomplicated than this simple example. This is a very time consumingroutine for current conditioning equipment, but a parallel network ofprocessors could be built into the previously mentioned data pipeline ina direct or a parallel manner depending on how fast the process is andhow much improvement in image results.

I. Neural Network Feature Detection

Feature detection is a very complex problem. The goal is to enable thecomputer to identify and label various layers of the artery andatheroma. A type of atheroma that is truly identifiable from the patterndisplayed is calcific plaque. This is unmistakable to the eye asindicated by a bright area with a blocked out region behind. Even thoughit is easy for a human to learn how to identify this feature of thedisplayed image of an atherosclerotic diseased artery, it would be verydifficult to write a program to identify and mark the region. Imageprocessing and technology for object and feature detection is currentlyin a very early stage as far a technical sophistication. Most computerobject detection is performed by performing a sequence of imagetransformation operations. The correct sequence is usually founditeratively by trying different combinations of the operations from alibrary of operations. Correct object detection is still a probabilisticevent where certain combinations have a higher hit ratio than others.

Other techniques could include doing fourier analysis or othermathematical modeling techniques to analyze the data to determine thedifferent features. From some of the published initial analysis of thematerials, it is seen that most of the materials that must bedistinguished from each other for feature detection have very closephysical properties. The acoustical properties that are of concern areacoustical impedance, impedance variation, texture, density, velocity,attenuation all a function of frequency. Even if there is some exhibitedvariation in the physical parameters, it is still a formidable task tocorrelate the variable from the information acquired from ultrasounddata.

Neural networks have been found to be very useful in solving a number ofvery difficult problems. They are being used currently for speechrecognition, autonomous vehicle guidance and many other complicatedproblems like this one where there are no clear and fast rules to modelthe inputs to the desired outputs.

Neural networks are a scalable architecture defined as a number ofweighted summing nodes organized in a layered manner. In FIG. 33 thereis depicted a diagram illustrating interconnections of a three layernetwork. Each layer node feeds its value forward as well as feedback toother layers. They can have any number of layers as well as any numberof nodes per layer.

The major advantage to neural networks is that the correct weighings onthe input nodes can be determined by a learning process. The network isprogrammed by exposing it to inputs and telling it the correct output.By doing this repeatedly with many examples the network can determinewhat the weighing values need to be to give the most accurate answer.

Determining the features in an ultrasound scan of an artery using neuralnetworks is the best approach. After the network has learned the correctresponses, a circuit could be developed to process the data in realtime. Initially the network would be designed to operate on the datagoing through the raw data pipeline. Here, the network could work on alimited number of vectors at one time as inputs. This would keep thecircuitry down to a practical level of parts. Handling the input rawdata from one vector would require handling 500 points. For a number ofcomplete vectors to be processed a large number of inputs result. A morereasonable approach is to use a network that processes a limited numberof points from each vector and use more vectors. A circuit handling 25radial points and 5 to 10 vectors could be developed with presentlyavailable hardware and yet contain all the neighborhood information fromthe acoustic beam that would be useful in reducing the data to an outputfeature.

J. Non-contacting Slip Rings

With mechanical rotating imaging transducers, one of the major concernsrelates to making a good electrical contact between the rotating driveshaft and the proximal electronics from the proximal end of the imagingdevice. In a first preferred embodiment in a 3 Fr size imager, describedabove, the transmission of the electrical signal from the imagerelongate shaft to the proximal electronics is provided by a mechanicalcontacting slip ring assembly 104. Although the slip ring assembly, asdescribed above, provides excellent transmission, in alternativeembodiments, it would be advantageous, and potentially a simplificationof the interface, if a non-contacting means were employed to coupleelectrical signal between the rotating and non-rotating parts. Twoalternative means for providing this transmission link are capacitivecoupling and magnetic coupling.

A first alternative embodiment of the signal coupling assembly is shownin FIG. 57. This embodiment employs capacitive coupling. Capacitivecoupling can be used when the capacitance is large enough between therotating and non-rotating contact rings. The capacitance is a functionof the surface area, the gap distance and the effective dielectricconstant. For a 30 Mhz signal, 100 pF would be more than enoughcapacitance to provide suitable coupling. A values greater or less thanthis would work also.

Capacitive contact rings 600 are shown to be longitudinally spaced,although alternatively, the rings 600 could be positioned radially. Ifpositioned radially, one ring would be placed on the inner diameter andthe other contact ring on the outer diameter of the assembly.

With either capacitive or magnetic non-contacting slip rings, themechanical energy is transferred by a keyed configuration or a frictionfit. There are other means that could be used to transfer the mechanicalenergy, for example by a magnetic drive. By making the rotating contactrings out of a magnetic material or by placing a permanent magnetic inthe assembly, the slip and drive shaft could be rotated without physicalcontact. A similar principle is used in stepper motors. There areseveral well known ways of generating an appropriate rotating magneticfield that the rotating contact rings would follow or of generating astepping multi-phase magnetic field that would drive the center throughthe rotation phases that following the stepper rotation.

FIG. 58 shows an embodiment of a magnetic non-contacting slip ringassembly 604. This alternative embodiment includes a rotating and anon-rotating transformer coil 608 and 610. The energy is transferred bymagnetic fields through the magnetic circuit. A consideration with thisembodiment is air gaps reducing the coupling between the two coils. Forthis reason, the gap area 612 is enlarged to minimize this problem.

K. EEPROM Catheter Information Storage

In present preferred and alternative embodiments of ultrasound imagingcatheters, there are numerous parameters that are device dependant.Currently, all imaging device dependant information is entered manuallyor by shunting contact pins to provide some device type information.These parameters may be as simple as device type, frequency, deviceserial number, and production information. Other imaging parameters thatare sensor dependant include those that would be used for a calibratedwaveform pulser or coefficients that describe the acoustic waveform thatwould be used in image enhancement routines, as described above. Thisinformation must be entered into the system before imaging begins.However, it is not very user-friendly to force the user to enter theinformation manually into the system.

A feature that can be incorporated into any of the embodiments discussedherein provides for automatic imager information entry. An embodimentincorporating this feature is shown in FIG. 59. The device dependantinformation is stored in a non-volatile storage medium 614. Such astorage medium is an EEPROM. In this embodiment, the information isavailable when the imaging catheter or imaging guide wire is pluggedinto the driving apparatus and control system 38. The means forconnection could be direct wiring or an isolated reading means could beused. A minimum of two wires are typically needed to transferinformation. Common serial EEPROM devices are available that operate offthree wires and have a wide range of storage capacity. Also potentiallyavailable but not as desirable is parallel access non-volatile storage.

Another easy method of entering this information is to provide aseparate data card or disk. This can be plugged into the system and thecomputer control can read the information before imaging begins.

L. Cath Lab System Integration

To use the imaging catheter or guide wire, drive and electricalconnections must be made. A setup for achieving and facilitating thistype of activity is illustrated in FIG. 60. FIG. 60 shows a motor box620 attached to the edge of a patient table 622. A gooseneck device 624extends the catheter connector over the table 622 and holds the imagingcatheter or guide wire in place. It is important to keep the imagingcatheter or guide wire straight while imaging. This gooseneck typedevice 624 allows movement back and forward easily to follow to doctoras the imaging is performed. Before and after imaging, the gooseneckdevice 624 and imaging catheter can be pushed back out of the way toeliminate some of the clutter on the patient table 622 as well as toprotect the imaging drive shaft from getting bent. This gooseneck device624 could have cables internal or external to its supporting structure.The goose neck device 624 preferably possesses a physical configurationand structure that can support a weight at a distance and be movedbetween two three-dimensional points.

Ultrasound imaging in catheter labs is currently performed by wheelingan ultrasound imaging system into the cath lab, setting up the systemand catheter and then imaging. There are other methods of systemintegration that depend on the catheter lab setup. In prior cath labsetups, a direct connection is made between the motor 630 andconditioning unit (MCU) 632. The motor is typically in a cabinet on acart and the MCU is mounted on the table. In this configuration, theproximal drive cable is laying across the floor and can be tripped on ifthe system is not next to the doctor. When the system is not next to thedoctor, the MCU should have a connector on the floor, the table orhanging from the ceiling.

According to a preferred setup, a connector 634 is mounted to the table622 so the MCU 632 cable follows the table 622 when it is moved. Thesystem also has a plug 636 and could be unplugged for portableconfigurations. In this configuration, there is also a connector forvideo input from the fluoroscope and video outputs for displaying on thedoctors' overhead monitor.

Other system configurations include a rack mount system integrated intoexisting or modified catheter lab control hardware. In thisconfiguration, the system is already on-line and when the doctor needsto perform an imaging procedure, the MCU 620 is mounted to the table 622and plugged in. At this time, imaging could begin. The externalcontroller could issue the system commands and the video outputs aremultiplexed and displayed at the doctors overhead screen.

Another alternative configuration provides for the system to be locatedwithin the MCU 620. This could be provided if the system electronicswere small enough to fit within a reasonable sized box to place on thetable rack. Here, there is a manual interface on the unit and that canbe operated remotely from an external controller. Also, a small monitorcan be provided internally, but the preferred method of viewing would beexternally on the overhead monitor. In this configuration, there is aplug for communications, video signals and power.

It is intended that the foregoing detailed description be regarded asillustrative rather than limiting and that it is understood that thefollowing claims including all equivalents are intended to define thescope of the invention.

I claim:
 1. A data processing architecture for use in an imaging deviceof the type having a video output, for ultrasonic imaging of smallvessels of a patient's body, the imaging device having a transducersized and adapted to be positioned intravascularly to scan small vesselsof the patient's body from within a small vessel, a drive cable having adistal end connected to the transducer and operable to transmitelectrical signals to and from the transducer and to rotate thetransducer to scan the vessel of the person's body with ultrasonicwaves, and a signal processor adapted for generating and receivingsignals to and from the transducer via the drive cable, the dataprocessing architecture comprising:a raw data pipeline coupled to saidsignal processor, adapted to apply a first process to said polarcoordinate data derived from the analog signals produced by saidtransducer; a graphics data pipeline coupled to said raw data pipelinethrough a look up table for converting polar coordinate data torectangular coordinate data and adapted to apply a second process tosaid rectangular coordinate data, thereby generating said video output.2. The apparatus of claim 1 wherein said first process is selected fromthe group consisting of: de-convolutions; fourier transform processing;or feature extraction.
 3. The apparatus of claim 1 wherein said secondprocess is selected from the group consisting of: edge detection; areamanipulation; or logical pixel edge smoothing.
 4. The apparatus of claim1 further comprising a buffer memory coupled to said raw data pipelinefor storing raw produced during the advancement of the imaging device tothe small vessels or withdrawal of the imaging device from the smallvessels.